--- a/browser/components/extensions/test/browser/browser_ext_browserAction_simple.js
+++ b/browser/components/extensions/test/browser/browser_ext_browserAction_simple.js
@@ -40,17 +40,19 @@ add_task(async function() {
   let waitForConsole = new Promise(resolve => {
     SimpleTest.monitorConsole(resolve, [
       {
         message: /Reading manifest: Warning processing browser_action.unrecognized_property: An unexpected property was found/,
       },
     ]);
   });
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   // Do this a few times to make sure the pop-up is reloaded each time.
   for (let i = 0; i < 3; i++) {
     clickBrowserAction(extension);
 
     let widget = getBrowserActionWidget(extension).forWindow(window);
     let image = getComputedStyle(widget.node).listStyleImage;
 

--- a/browser/components/extensions/test/browser/browser_ext_commands_onCommand.js
+++ b/browser/components/extensions/test/browser/browser_ext_commands_onCommand.js
@@ -233,18 +233,19 @@ add_task(async function test_user_define
   SimpleTest.waitForExplicitFinish();
   let waitForConsole = new Promise(resolve => {
     SimpleTest.monitorConsole(resolve, [
       {
         message: /Reading manifest: Warning processing commands.*.unrecognized_property: An unexpected property was found/,
       },
     ]);
   });
-
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   await extension.awaitMessage("ready");
 
   async function runTest(window) {
     for (let testCommand of testCommands) {
       if (testCommand.shortcutMac && !testCommand.shortcut && !isMac) {
         continue;
       }
       EventUtils.synthesizeKey(testCommand.key, testCommand.modifiers, window);
@@ -318,17 +319,21 @@ add_task(async function test_user_define
   // Test that given permission the keyset is added to the private window.
   extension = ExtensionTestUtils.loadExtension({
     manifest: {
       commands: commands,
     },
     incognitoOverride: "spanning",
     background,
   });
+
+  // unrecognized_property in manifest triggers warning.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   await extension.awaitMessage("ready");
   keysetID = `ext-keyset-id-${makeWidgetId(extension.id)}`;
 
   keyset = win1.document.getElementById(keysetID);
   ok(keyset != null, "Expected keyset to exist on win1");
   is(
     keyset.children.length,
     expectedCommandsRegistered,

--- a/browser/components/extensions/test/browser/browser_ext_pageAction_simple.js
+++ b/browser/components/extensions/test/browser/browser_ext_pageAction_simple.js
@@ -50,17 +50,19 @@ add_task(async function test_pageAction_
   let waitForConsole = new Promise(resolve => {
     SimpleTest.monitorConsole(resolve, [
       {
         message: /Reading manifest: Warning processing page_action.unrecognized_property: An unexpected property was found/,
       },
     ]);
   });
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   await extension.awaitMessage("page-action-shown");
 
   let elem = await getPageActionButton(extension);
   let parent = window.document.getElementById("page-action-buttons");
   is(
     elem && elem.parentNode,
     parent,
     `pageAction pinned to urlbar ${elem.parentNode.getAttribute("id")}`

--- a/devtools/client/aboutdebugging/test/browser/browser_aboutdebugging_addons_warnings.js
+++ b/devtools/client/aboutdebugging/test/browser/browser_aboutdebugging_addons_warnings.js
@@ -9,27 +9,29 @@ Services.scriptloader.loadSubScript(CHRO
 // Test that extension warnings are displayed in about:debugging.
 add_task(async function() {
   const EXTENSION_NAME = "Temporary web extension";
   const EXTENSION_ID = "test-devtools@mozilla.org";
 
   const { document, tab, window } = await openAboutDebugging();
   await selectThisFirefoxPage(document, window.AboutDebugging.store);
 
+  await pushPref("extensions.webextensions.warnings-as-errors", false);
   await installTemporaryExtensionFromXPI(
     {
       id: EXTENSION_ID,
       name: EXTENSION_NAME,
       extraProperties: {
         // This property is not expected in the manifest and should trigger a warning!
         wrongProperty: {},
       },
     },
     document
   );
+  await SpecialPowers.popPrefEnv();
 
   info("Wait until a debug target item appears");
   await waitUntil(() => findDebugTargetByText(EXTENSION_NAME, document));
   const target = findDebugTargetByText(EXTENSION_NAME, document);
 
   const warningMessage = target.querySelector(".qa-message");
   ok(
     !!warningMessage,

--- a/devtools/client/aboutdebugging/test/browser/browser_aboutdebugging_message_close.js
+++ b/devtools/client/aboutdebugging/test/browser/browser_aboutdebugging_message_close.js
@@ -49,27 +49,29 @@ async function testCloseMessageWithButto
   info("Click on the button and wait for the message to disappear");
   EventUtils.synthesizeMouse(closeButton, 1, 1, {}, doc.defaultView);
 
   const target = findDebugTargetByText(EXTENSION_NAME, doc);
   await waitUntil(() => target.querySelector(".qa-message") === null);
 }
 
 async function installExtensionWithWarning(doc) {
+  await pushPref("extensions.webextensions.warnings-as-errors", false);
   await installTemporaryExtensionFromXPI(
     {
       id: EXTENSION_ID,
       name: EXTENSION_NAME,
       extraProperties: {
         // This property is not expected in the manifest and should trigger a warning!
         wrongProperty: {},
       },
     },
     doc
   );
+  await SpecialPowers.popPrefEnv();
 
   info("Wait until a debug target item appears");
   await waitUntil(() => findDebugTargetByText(EXTENSION_NAME, doc));
 
   const target = findDebugTargetByText(EXTENSION_NAME, doc);
   const warningMessage = target.querySelector(".qa-message");
   ok(
     !!warningMessage,

--- a/testing/mochitest/tests/SimpleTest/ExtensionTestUtils.js
+++ b/testing/mochitest/tests/SimpleTest/ExtensionTestUtils.js
@@ -133,9 +133,23 @@ ExtensionTestUtils.loadExtension = funct
         SimpleTest.is(actual, msg, "test result correct");
       }
       return actual;
     });
   };
 
   SimpleTest.info(`Extension loaded`);
   return extension;
-}
+};
+
+ExtensionTestUtils.failOnSchemaWarnings = (warningsAsErrors = true) => {
+  let prefName = "extensions.webextensions.warnings-as-errors";
+  SpecialPowers.setBoolPref(prefName, warningsAsErrors);
+  if (!warningsAsErrors) {
+    let registerCleanup;
+    if (typeof registerCleanupFunction != "undefined") {
+      registerCleanup = registerCleanupFunction;
+    } else {
+      registerCleanup = SimpleTest.registerCleanupFunction.bind(SimpleTest);
+    }
+    registerCleanup(() => SpecialPowers.setBoolPref(prefName, true));
+  }
+};

--- a/testing/profiles/common/user.js
+++ b/testing/profiles/common/user.js
@@ -36,16 +36,18 @@ user_pref("dom.send_after_paint_to_conte
 // Only load extensions from the application and user profile
 // AddonManager.SCOPE_PROFILE + AddonManager.SCOPE_APPLICATION
 user_pref("extensions.enabledScopes", 5);
 user_pref("extensions.legacy.enabled", true);
 // Turn off extension updates so they don't bother tests
 user_pref("extensions.update.enabled", false);
 // Prevent network access for recommendations by default. The payload is {"results":[]}.
 user_pref("extensions.getAddons.discovery.api_url", "data:;base64,eyJyZXN1bHRzIjpbXX0%3D");
+// Treat WebExtension API/schema warnings as errors.
+user_pref("extensions.webextensions.warnings-as-errors", true);
 // Disable useragent updates.
 user_pref("general.useragent.updates.enabled", false);
 // Ensure WR doesn't get enabled in tests unless we do it explicitly with the MOZ_WEBRENDER envvar.
 user_pref("gfx.webrender.all.qualified", false);
 user_pref("hangmonitor.timeout", 0); // no hang monitor
 user_pref("media.gmp-manager.updateEnabled", false);
 // Don't do network connections for mitm priming
 user_pref("security.certerrors.mitm.priming.enabled", false);

--- a/testing/profiles/xpcshell/user.js
+++ b/testing/profiles/xpcshell/user.js
@@ -1,15 +1,17 @@
 // Base preferences file used by the xpcshell harness
 /* globals user_pref */
 /* eslint quotes: 0 */
 user_pref("app.normandy.api_url", "https://%(server)s/selfsupport-dummy/");
 user_pref("browser.safebrowsing.downloads.remote.url", "https://%(server)s/safebrowsing-dummy");
 user_pref("browser.search.geoip.url", "https://%(server)s/geoip-dummy");
 user_pref("extensions.systemAddon.update.url", "http://%(server)s/dummy-system-addons.xml");
+// Treat WebExtension API/schema warnings as errors.
+user_pref("extensions.webextensions.warnings-as-errors", true);
 // Always use network provider for geolocation tests
 // so we bypass the OSX dialog raised by the corelocation provider
 user_pref("geo.provider.testing", true);
 user_pref("media.gmp-manager.updateEnabled", false);
 user_pref("media.gmp-manager.url.override", "http://%(server)s/dummy-gmp-manager.xml");
 user_pref("toolkit.telemetry.server", "https://%(server)s/telemetry-dummy");
 // Prevent Remote Settings to issue non local connections.
 user_pref("services.settings.server", "http://localhost/remote-settings-dummy/v1");

--- a/toolkit/components/extensions/ExtensionXPCShellUtils.jsm
+++ b/toolkit/components/extensions/ExtensionXPCShellUtils.jsm
@@ -950,16 +950,26 @@ var ExtensionTestUtils = {
   },
 
   // Create a wrapper for a webextension that will be installed
   // by some external process (e.g., Normandy)
   expectExtension(id) {
     return new ExternallyInstalledWrapper(this.currentScope, id);
   },
 
+  failOnSchemaWarnings(warningsAsErrors = true) {
+    let prefName = "extensions.webextensions.warnings-as-errors";
+    Services.prefs.setBoolPref(prefName, warningsAsErrors);
+    if (!warningsAsErrors) {
+      this.currentScope.registerCleanupFunction(() => {
+        Services.prefs.setBoolPref(prefName, true);
+      });
+    }
+  },
+
   get remoteContentScripts() {
     return REMOTE_CONTENT_SCRIPTS;
   },
 
   set remoteContentScripts(val) {
     REMOTE_CONTENT_SCRIPTS = !!val;
   },
 

--- a/toolkit/components/extensions/Schemas.jsm
+++ b/toolkit/components/extensions/Schemas.jsm
@@ -45,16 +45,23 @@ XPCOMUtils.defineLazyServiceGetter(
 );
 
 XPCOMUtils.defineLazyGetter(
   this,
   "StartupCache",
   () => ExtensionParent.StartupCache
 );
 
+XPCOMUtils.defineLazyPreferenceGetter(
+  this,
+  "treatWarningsAsErrors",
+  "extensions.webextensions.warnings-as-errors",
+  false
+);
+
 var EXPORTED_SYMBOLS = ["SchemaRoot", "Schemas"];
 
 const KEY_CONTENT_SCHEMAS = "extensions-framework/schemas/content";
 const KEY_PRIVILEGED_SCHEMAS = "extensions-framework/schemas/privileged";
 
 const { DEBUG } = AppConstants;
 
 const isParentProcess =
@@ -1188,17 +1195,41 @@ class Entry {
           value = JSON.stringify(value);
         } catch (e) {
           value = String(value);
         }
         message = message.replace(/\$\{value\}/g, () => value);
       }
     }
 
-    context.logError(context.makeError(message, { warning: true }));
+    this.logWarning(context, message);
+  }
+
+  /**
+   * @param {Context} context
+   * @param {string} warningMessage
+   */
+  logWarning(context, warningMessage) {
+    let error = context.makeError(warningMessage, { warning: true });
+    context.logError(error);
+
+    if (treatWarningsAsErrors) {
+      // This pref is false by default, and true by default in tests to
+      // discourage the use of deprecated APIs in our unit tests.
+      // If a warning is an expected part of a test, temporarily set the pref
+      // to false, e.g. with the ExtensionTestUtils.failOnSchemaWarnings helper.
+      Services.console.logStringMessage(
+        "Treating warning as error because the preference " +
+          "extensions.webextensions.warnings-as-errors is set to true"
+      );
+      if (typeof error === "string") {
+        error = new Error(error);
+      }
+      throw error;
+    }
   }
 
   /**
    * Checks whether the entry is deprecated and, if so, logs a
    * deprecation message.
    *
    * @param {Context} context
    * @param {value} [value]
@@ -1869,17 +1900,17 @@ class ObjectType extends Type {
         `contain the required "${prop}" property`
       );
     } else if (optional !== "omit-key-if-missing") {
       result[prop] = propType.default;
     }
 
     if (error) {
       if (onError == "warn") {
-        context.logError(forceString(error.error));
+        this.logWarning(context, forceString(error.error));
       } else if (onError != "ignore") {
         throw error;
       }
 
       result[prop] = propType.default;
     }
   }
 
@@ -2167,17 +2198,17 @@ class ArrayType extends Type {
 
     let result = [];
     for (let [i, element] of value.entries()) {
       element = context.withPath(String(i), () =>
         this.itemType.normalize(element, context)
       );
       if (element.error) {
         if (this.onError == "warn") {
-          context.logError(forceString(element.error));
+          this.logWarning(context, forceString(element.error));
         } else if (this.onError != "ignore") {
           return element;
         }
         continue;
       }
       result.push(element.value);
     }
 

--- a/toolkit/components/extensions/test/browser/browser_ext_themes_lwtsupport.js
+++ b/toolkit/components/extensions/test/browser/browser_ext_themes_lwtsupport.js
@@ -1,14 +1,17 @@
 "use strict";
 
 const DEFAULT_THEME_BG_COLOR = "rgb(255, 255, 255)";
 const DEFAULT_THEME_TEXT_COLOR = "rgb(0, 0, 0)";
 
 add_task(async function test_deprecated_LWT_properties_ignored() {
+  // This test uses deprecated theme properties, so warnings are expected.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
+
   let extension = ExtensionTestUtils.loadExtension({
     manifest: {
       theme: {
         images: {
           headerURL: "image1.png",
         },
         colors: {
           accentcolor: ACCENT_COLOR,

--- a/toolkit/components/extensions/test/mochitest/test_chrome_ext_contentscript_unrecognizedprop_warning.html
+++ b/toolkit/components/extensions/test/mochitest/test_chrome_ext_contentscript_unrecognizedprop_warning.html
@@ -55,17 +55,19 @@ add_task(async function test_contentscri
 
   SimpleTest.waitForExplicitFinish();
   let waitForConsole = new Promise(resolve => {
     SimpleTest.monitorConsole(resolve, [{
       message: /Reading manifest: Warning processing content_scripts.*.unrecognized_property: An unexpected property was found/,
     }]);
   });
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   window.open(`${BASE}/file_sample.html`);
 
   await Promise.all([extension.awaitFinish("content-script-loaded")]);
   info("test page loaded");
 
   await extension.unload();
 

--- a/toolkit/components/extensions/test/xpcshell/test_ext_eventpage_warning.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_eventpage_warning.js
@@ -1,11 +1,13 @@
 "use strict";
 
 AddonTestUtils.init(this);
+// This test expects and checks deprecation warnings.
+ExtensionTestUtils.failOnSchemaWarnings(false);
 
 function createEventPageExtension(eventPage) {
   return ExtensionTestUtils.loadExtension({
     manifest: {
       background: eventPage,
     },
     files: {
       "event_page_script.js"() {

--- a/toolkit/components/extensions/test/xpcshell/test_ext_manifest.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_manifest.js
@@ -70,17 +70,19 @@ add_task(async function test_manifest_wa
         wrong_prop: true,
       },
     },
     files: {
       "bg.js": "",
     },
   });
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   // Retrieve the warning message collected by the Extension class
   // packagingWarning method.
   const { warnings } = extension.extension;
   equal(warnings.length, 1, "Got the expected number of manifest warnings");
 
   const expectedMessage =
     "Reading manifest: Warning processing background.wrong_prop";

--- a/toolkit/components/extensions/test/xpcshell/test_ext_manifest_content_security_policy.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_manifest_content_security_policy.js
@@ -10,19 +10,21 @@ add_task(async function test_manifest_cs
   equal(normalized.error, undefined, "Should not have an error");
   equal(normalized.errors.length, 0, "Should not have warnings");
   equal(
     normalized.value.content_security_policy,
     "script-src 'self'; object-src 'none'",
     "Should have the expected poilcy string"
   );
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   normalized = await ExtensionTestUtils.normalizeManifest({
     content_security_policy: "object-src 'none'",
   });
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   equal(normalized.error, undefined, "Should not have an error");
 
   Assert.deepEqual(
     normalized.errors,
     [
       "Error processing content_security_policy: SyntaxError: Policy is missing a required \u2018script-src\u2019 directive",
     ],

--- a/toolkit/components/extensions/test/xpcshell/test_ext_manifest_themes.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_manifest_themes.js
@@ -24,10 +24,12 @@ async function test_theme_property(prope
     equal(normalized.errors.length, 0, "Should have a warning");
   }
   equal(normalized.error, undefined, "Should not have an error");
 }
 
 add_task(async function test_manifest_themes() {
   await test_theme_property("images");
   await test_theme_property("colors");
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await test_theme_property("unrecognized_key");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 });

--- a/toolkit/components/extensions/test/xpcshell/test_ext_permission_warnings.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_permission_warnings.js
@@ -13,17 +13,20 @@ if (AppConstants.MOZ_APP_NAME == "thunde
   bundle = Services.strings.createBundle(
     "chrome://browser/locale/browser.properties"
   );
 }
 const DUMMY_APP_NAME = "Dummy brandName";
 
 async function getManifestPermissions(extensionData) {
   let extension = ExtensionTestCommon.generate(extensionData);
+  // Some tests contain invalid permissions; ignore the warnings about their invalidity.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.loadManifest();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   return extension.manifestPermissions;
 }
 
 function getPermissionWarnings(manifestPermissions) {
   let info = {
     permissions: manifestPermissions,
     appName: DUMMY_APP_NAME,
   };

--- a/toolkit/components/extensions/test/xpcshell/test_ext_proxy_auth.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_proxy_auth.js
@@ -110,18 +110,21 @@ add_task(async function test_webRequest_
     );
 
     await browser.proxy.register("proxy.js");
     browser.test.sendMessage("pac-ready");
   }
 
   let handlingExt = getExtension(background);
 
+  // proxy.register is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await handlingExt.startup();
   await handlingExt.awaitMessage("pac-ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   authManager.clearAll();
 
   let contentPage = await ExtensionTestUtils.loadContentPage(
     `http://mozilla.org/`
   );
 
   await handlingExt.awaitMessage("done");
@@ -152,18 +155,21 @@ add_task(async function test_webRequest_
     );
 
     await browser.proxy.register("proxy.js");
     browser.test.sendMessage("pac-ready");
   }
 
   let handlingExt = getExtension(background);
 
+  // proxy.register is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await handlingExt.startup();
   await handlingExt.awaitMessage("pac-ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   function fetch(url) {
     return new Promise((resolve, reject) => {
       let xhr = new XMLHttpRequest();
       xhr.mozBackgroundRequest = true;
       xhr.open("GET", url);
       xhr.onload = () => {
         resolve(xhr.responseText);

--- a/toolkit/components/extensions/test/xpcshell/test_ext_proxy_socks.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_proxy_socks.js
@@ -538,18 +538,21 @@ add_task(async function test_webRequest_
             port: ${socksServer.listener.localPort},
             username: "foo",
             password: "bar",
           }];
         }`,
     },
   });
 
+  // proxy.register is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await handlingExt.startup();
   await handlingExt.awaitMessage("pac-ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   let contentPage = await ExtensionTestUtils.loadContentPage(
     `http://localhost/`
   );
 
   await handlingExt.awaitMessage("done");
   await contentPage.close();
   await handlingExt.unload();

--- a/toolkit/components/extensions/test/xpcshell/test_ext_schema.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_schema.js
@@ -28,29 +28,47 @@ add_task(async function testEmptySchema(
     },
   });
 
   await extension.startup();
   await extension.awaitFinish("schema");
   await extension.unload();
 });
 
+add_task(async function test_warnings_as_errors() {
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: { unrecognized_property_that_should_be_treated_as_a_warning: 1 },
+  });
+
+  // Tests should be run with extensions.webextensions.warnings-as-errors=true
+  // by default, and prevent extensions with manifest warnings from loading.
+  await Assert.rejects(
+    extension.startup(),
+    /unrecognized_property_that_should_be_treated_as_a_warning/,
+    "extension with invalid manifest should not load if warnings-as-errors=true"
+  );
+  // When ExtensionTestUtils.failOnSchemaWarnings(false) is called, startup is
+  // expected to succeed, as shown by the next "testUnknownProperties" test.
+});
+
 add_task(async function testUnknownProperties() {
   let extension = ExtensionTestUtils.loadExtension({
     manifest: {
       permissions: ["unknownPermission"],
 
       unknown_property: {},
     },
 
     background() {},
   });
 
   let { messages } = await promiseConsoleOutput(async () => {
+    ExtensionTestUtils.failOnSchemaWarnings(false);
     await extension.startup();
+    ExtensionTestUtils.failOnSchemaWarnings(true);
   });
 
   AddonTestUtils.checkMessages(messages, {
     expected: [
       { message: /processing permissions\.0: Value "unknownPermission"/ },
       {
         message: /processing unknown_property: An unexpected property was found in the WebExtension manifest/,
       },

--- a/toolkit/components/extensions/test/xpcshell/test_ext_schemas.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_schemas.js
@@ -946,17 +946,19 @@ add_task(async function() {
   checkErrors([]);
 
   root.testing.errors({ default: "0123", ignore: "x123", warn: "0123" });
   verify("call", "testing", "errors", [
     { default: "0123", ignore: null, warn: "0123" },
   ]);
   checkErrors([]);
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   root.testing.errors({ default: "0123", ignore: "0123", warn: "x123" });
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   verify("call", "testing", "errors", [
     { default: "0123", ignore: "0123", warn: null },
   ]);
   checkErrors(['String "x123" must match /^\\d+$/']);
 
   root.testing.onFoo.addListener(f);
   Assert.equal(
     JSON.stringify(tallied.slice(0, -1)),
@@ -1247,16 +1249,19 @@ let deprecatedJson = [
         type: "function",
         deprecated: "This event does not work",
       },
     ],
   },
 ];
 
 add_task(async function testDeprecation() {
+  // This whole test expects deprecation warnings.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
+
   let url = "data:," + JSON.stringify(deprecatedJson);
   Schemas._rootSchema = null;
   await Schemas.load(url);
 
   let root = {};
   Schemas.inject(root, wrapper);
 
   talliedErrors.length = 0;
@@ -1301,16 +1306,24 @@ add_task(async function testDeprecation(
   root.deprecated.onDeprecated.addListener(() => {});
   checkErrors(["This event does not work"]);
 
   root.deprecated.onDeprecated.removeListener(() => {});
   checkErrors(["This event does not work"]);
 
   root.deprecated.onDeprecated.hasListener(() => {});
   checkErrors(["This event does not work"]);
+
+  ExtensionTestUtils.failOnSchemaWarnings(true);
+
+  Assert.throws(
+    () => root.deprecated.onDeprecated.hasListener(() => {}),
+    /This event does not work/,
+    "Deprecation warning with extensions.webextensions.warnings-as-errors=true"
+  );
 });
 
 let choicesJson = [
   {
     namespace: "choices",
 
     types: [],
 

--- a/toolkit/components/extensions/test/xpcshell/test_ext_unknown_permissions.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_unknown_permissions.js
@@ -1,10 +1,13 @@
 "use strict";
 
+// This test expects and checks warnings for unknown permissions.
+ExtensionTestUtils.failOnSchemaWarnings(false);
+
 add_task(async function test_unknown_permissions() {
   let extension = ExtensionTestUtils.loadExtension({
     manifest: {
       permissions: [
         "activeTab",
         "fooUnknownPermission",
         "http://*/",
         "chrome://favicon/",

--- a/toolkit/components/extensions/test/xpcshell/test_ext_userScripts.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_userScripts.js
@@ -396,17 +396,19 @@ add_task(async function test_cached_user
       },
     },
     background,
     files: {
       "api-script.js": apiScript,
     },
   });
 
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   await extension.awaitMessage("user-script-registered");
 
   let url = `${BASE_URL}/file_sample.html`;
   let contentPage = await ExtensionTestUtils.loadContentPage(url);
 
   let msg = await extension.awaitMessage("user-script-loaded");
   Assert.deepEqual(
     msg,

--- a/toolkit/components/extensions/test/xpcshell/test_proxy_incognito.js
+++ b/toolkit/components/extensions/test/xpcshell/test_proxy_incognito.js
@@ -128,18 +128,21 @@ add_task(async function test_incognito_p
         function FindProxyForURL() {
           // Shoot a message off to the background.
           browser.runtime.sendMessage("incognito fail");
           return null;
         }
       }),
     },
   });
+  // proxy.register is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
   await extension.awaitMessage("ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   // This extension will succeed if it gets a request
   let pb_extension = ExtensionTestUtils.loadExtension({
     incognitoOverride: "spanning",
     manifest: {
       permissions: ["proxy"],
     },
     async background() {
@@ -156,18 +159,21 @@ add_task(async function test_incognito_p
         function FindProxyForURL() {
           // Shoot a message off to the background.
           browser.runtime.sendMessage("success");
           return null;
         }
       }),
     },
   });
+  // proxy.register is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await pb_extension.startup();
   await pb_extension.awaitMessage("ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   let finished = pb_extension.awaitFinish("success");
   let contentPage = await ExtensionTestUtils.loadContentPage(
     "http://example.com/dummy",
     { privateBrowsing: true }
   );
   await finished;
 

--- a/toolkit/components/extensions/test/xpcshell/test_proxy_scripts.js
+++ b/toolkit/components/extensions/test/xpcshell/test_proxy_scripts.js
@@ -52,18 +52,21 @@ async function testProxyScript(script, e
       });
     },
     files: {
       "proxy.js": scriptData,
     },
   };
 
   let extension = ExtensionTestUtils.loadExtension(extensionData);
+  // proxy.register and proxy.onProxyError are deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
   await extension.awaitMessage("ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   let errorWait = extension.awaitMessage("proxy-error-received");
 
   let proxyInfo = await getProxyInfo();
 
   let error = await errorWait;
   equal(error.message, expected.message, "Correct error message received");
   if (!expected.proxyInfo) {
@@ -77,18 +80,21 @@ async function testProxyScript(script, e
   if (expected.errorInfo) {
     ok(error.fileName.includes("proxy.js"), "Error should include file name");
     equal(error.lineNumber, 3, "Error should include line number");
     ok(
       error.stack.includes("proxy.js:3:9"),
       "Error should include stack trace"
     );
   }
+  // proxy.unregister is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   extension.sendMessage("unregister-proxy-script");
   await extension.awaitFinish("proxy");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   await extension.unload();
 }
 
 add_task(async function test_invalid_FindProxyForURL_function() {
   await testProxyScript(() => {}, {
     message: "The proxy script must define FindProxyForURL as a function",
   });
 
@@ -157,18 +163,21 @@ async function getExtension(proxyResult)
     files: {
       "proxy.js": `
         function FindProxyForURL(url, host) {
           return ${proxyResult};
         }`,
     },
   };
   let extension = ExtensionTestUtils.loadExtension(extensionData);
+  // proxy.register is deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
   await extension.awaitMessage("ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
   return extension;
 }
 
 add_task(async function test_passthrough() {
   let ext1 = await getExtension(null);
   let ext2 = await getExtension('"PROXY 1.2.3.4:8888"');
 
   let proxyInfo = await getProxyInfo();

--- a/toolkit/components/extensions/test/xpcshell/test_proxy_scripts_deprecated.js
+++ b/toolkit/components/extensions/test/xpcshell/test_proxy_scripts_deprecated.js
@@ -1,11 +1,13 @@
 "use strict";
 
 AddonTestUtils.init(this);
+// This test expects and checks deprecation messages.
+ExtensionTestUtils.failOnSchemaWarnings(false);
 
 add_task(async function test_proxy_deprecation_messages() {
   let extension = ExtensionTestUtils.loadExtension({
     manifest: {
       permissions: ["proxy"],
     },
     background() {
       async function callProxyAPIs() {

--- a/toolkit/components/extensions/test/xpcshell/test_proxy_scripts_results.js
+++ b/toolkit/components/extensions/test/xpcshell/test_proxy_scripts_results.js
@@ -46,18 +46,21 @@ add_task(async function setup() {
             settings.proxy = msg.proxy;
             return Promise.resolve(settings.proxy);
           }
         });
       `,
     },
   };
   extension = ExtensionTestUtils.loadExtension(extensionData);
+  // proxy.register and proxy.onProxyError are deprecated - bug 1443259.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
   await extension.awaitMessage("ready");
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 });
 
 async function setupProxyScript(proxy) {
   extension.sendMessage("set-proxy", { proxy });
   let proxyInfoSent = await extension.awaitMessage("proxy-set");
   deepEqual(proxyInfoSent, proxy, "got back proxy data from proxy script");
 }
 

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
@@ -383,32 +383,34 @@ add_task(async function developerEmpty()
       homepageURL: "https://example.net",
     });
 
     await addon.uninstall();
   }
 });
 
 add_task(async function authorNotString() {
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   for (let author of [{}, [], 42]) {
     let addon = await promiseInstallWebExtension({
       manifest: {
         author,
         manifest_version: 2,
         name: "Web Extension Name",
         version: "1.0",
       },
     });
 
     checkAddon(ID, addon, {
       creator: null,
     });
 
     await addon.uninstall();
   }
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 });
 
 add_task(async function testThemeExtension() {
   let addon = await promiseInstallWebExtension({
     manifest: {
       author: "Some author",
       manifest_version: 2,
       name: "Web Extension Name",
@@ -429,25 +431,27 @@ add_task(async function testThemeExtensi
     type: "theme",
     isWebExtension: true,
     signedState: AddonManager.SIGNEDSTATE_PRIVILEGED,
   });
 
   await addon.uninstall();
 
   // Also test one without a proper 'theme' section.
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   addon = await promiseInstallWebExtension({
     manifest: {
       author: "Some author",
       manifest_version: 2,
       name: "Web Extension Name",
       version: "1.0",
       theme: null,
     },
   });
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   checkAddon(ID, addon, {
     type: "extension",
     isWebExtension: true,
   });
 
   await addon.uninstall();
 });

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_install.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_install.js
@@ -649,15 +649,17 @@ add_task(async function test_non_gecko_b
       },
     },
   };
 
   const extension = ExtensionTestUtils.loadExtension({
     manifest,
     useAddonManager: "temporary",
   });
+  ExtensionTestUtils.failOnSchemaWarnings(false);
   await extension.startup();
+  ExtensionTestUtils.failOnSchemaWarnings(true);
 
   const addon = await promiseAddonByID(ID);
   notEqual(addon, null, "Add-on is installed");
 
   await extension.unload();
 });