--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -296,16 +296,19 @@ pref("browser.startup.firstrunSkipsHomep
 #if defined(XP_WIN)
 #ifdef NIGHTLY_BUILD
 pref("browser.startup.preXulSkeletonUI", true);
 #else
 pref("browser.startup.preXulSkeletonUI", false);
 #endif
 #endif
 
+// Show an upgrade dialog on major upgrades.
+pref("browser.startup.upgradeDialog.enabled", true);
+
 // Don't create the hidden window during startup on
 // platforms that don't always need it (Win/Linux).
 pref("toolkit.lazyHiddenWindow", true);
 
 pref("browser.chrome.site_icons", true);
 // browser.warnOnQuit == false will override all other possible prompts when quitting or restarting
 pref("browser.warnOnQuit", true);
 

--- a/browser/components/BrowserContentHandler.jsm
+++ b/browser/components/BrowserContentHandler.jsm
@@ -87,17 +87,17 @@ function resolveURIInternal(aCmdLine, aA
   } catch (e) {
     Cu.reportError(e);
   }
 
   return uri;
 }
 
 let gKiosk = false;
-
+let gMajorUpgrade = false;
 var gFirstWindow = false;
 
 const OVERRIDE_NONE = 0;
 const OVERRIDE_NEW_PROFILE = 1;
 const OVERRIDE_NEW_MSTONE = 2;
 const OVERRIDE_NEW_BUILD_ID = 3;
 const OVERRIDE_ALTERNATE_PROFILE = 4;
 /**
@@ -137,16 +137,19 @@ function needHomepageOverride(prefb) {
 
   if (mstone != savedmstone) {
     // Bug 462254. Previous releases had a default pref to suppress the EULA
     // agreement if the platform's installer had already shown one. Now with
     // about:rights we've removed the EULA stuff and default pref, but we need
     // a way to make existing profiles retain the default that we removed.
     if (savedmstone) {
       prefb.setBoolPref("browser.rights.3.shown", true);
+
+      // Remember that we saw a major version change.
+      gMajorUpgrade = true;
     }
 
     prefb.setCharPref("browser.startup.homepage_override.mstone", mstone);
     prefb.setCharPref("browser.startup.homepage_override.buildID", buildID);
     return savedmstone ? OVERRIDE_NEW_MSTONE : OVERRIDE_NEW_PROFILE;
   }
 
   if (buildID != savedBuildID) {
@@ -832,16 +835,24 @@ nsBrowserContentHandler.prototype = {
 
     return this.mFeatures;
   },
 
   get kiosk() {
     return gKiosk;
   },
 
+  get majorUpgrade() {
+    return gMajorUpgrade;
+  },
+
+  set majorUpgrade(val) {
+    gMajorUpgrade = val;
+  },
+
   /* nsIContentHandler */
 
   handleContent: function bch_handleContent(contentType, context, request) {
     const NS_ERROR_WONT_HANDLE_CONTENT = 0x805d0001;
 
     try {
       var webNavInfo = Cc["@mozilla.org/webnavigation-info;1"].getService(
         Ci.nsIWebNavigationInfo

--- a/browser/components/BrowserGlue.jsm
+++ b/browser/components/BrowserGlue.jsm
@@ -50,16 +50,17 @@ XPCOMUtils.defineLazyModuleGetters(this,
   FirefoxMonitor: "resource:///modules/FirefoxMonitor.jsm",
   FxAccounts: "resource://gre/modules/FxAccounts.jsm",
   HomePage: "resource:///modules/HomePage.jsm",
   Integration: "resource://gre/modules/Integration.jsm",
   Log: "resource://gre/modules/Log.jsm",
   LoginBreaches: "resource:///modules/LoginBreaches.jsm",
   NetUtil: "resource://gre/modules/NetUtil.jsm",
   NewTabUtils: "resource://gre/modules/NewTabUtils.jsm",
+  NimbusFeatures: "resource://nimbus/ExperimentAPI.jsm",
   Normandy: "resource://normandy/Normandy.jsm",
   OS: "resource://gre/modules/osfile.jsm",
   OsEnvironment: "resource://gre/modules/OsEnvironment.jsm",
   PageActions: "resource:///modules/PageActions.jsm",
   PageThumbs: "resource://gre/modules/PageThumbs.jsm",
   PdfJs: "resource://pdf.js/PdfJs.jsm",
   PermissionUI: "resource:///modules/PermissionUI.jsm",
   PlacesBackups: "resource://gre/modules/PlacesBackups.jsm",
@@ -108,22 +109,20 @@ let initializedModules = {};
 ].forEach(([name, resource, init]) => {
   XPCOMUtils.defineLazyGetter(this, name, () => {
     ChromeUtils.import(resource, initializedModules);
     initializedModules[name][init]();
     return initializedModules[name];
   });
 });
 
-XPCOMUtils.defineLazyServiceGetter(
-  this,
-  "PushService",
-  "@mozilla.org/push/Service;1",
-  "nsIPushService"
-);
+XPCOMUtils.defineLazyServiceGetters(this, {
+  BrowserHandler: ["@mozilla.org/browser/clh;1", "nsIBrowserHandler"],
+  PushService: ["@mozilla.org/push/Service;1", "nsIPushService"],
+});
 
 const PREF_PDFJS_ISDEFAULT_CACHE_STATE = "pdfjs.enabledCache.state";
 
 /**
  * Fission-compatible JSProcess implementations.
  * Each actor options object takes the form of a ProcessActorOptions dictionary.
  * Detailed documentation of these options is in dom/docs/ipc/jsactors.rst,
  * available at https://firefox-source-docs.mozilla.org/dom/ipc/jsactors.html
@@ -3755,51 +3754,94 @@ BrowserGlue.prototype = {
   },
 
   _showUpgradeDialog() {
     BrowserWindowTracker.getTopWindow().gDialogBox.open(
       "chrome://browser/content/upgradeDialog.html"
     );
   },
 
-  _maybeShowDefaultBrowserPrompt() {
-    Promise.all([
-      DefaultBrowserCheck.willCheckDefaultBrowser(/* isStartupCheck */ true),
-      ExperimentAPI.ready,
-    ]).then(async ([willPrompt]) => {
-      let { DefaultBrowserNotification } = ChromeUtils.import(
-        "resource:///actors/AboutNewTabParent.jsm",
-        {}
-      );
-      let isFeatureEnabled = ExperimentAPI.getExperiment({
-        featureId: "infobar",
-      })?.branch.feature.enabled;
-      if (willPrompt) {
-        // Prevent the related notification from appearing and
-        // show the modal prompt.
-        DefaultBrowserNotification.notifyModalDisplayed();
+  async _maybeShowDefaultBrowserPrompt() {
+    // Highest priority is the upgrade dialog, which can include a "primary
+    // browser" request and is limited in various ways, e.g., major upgrades.
+    const dialogVersion = 89;
+    const dialogVersionPref = "browser.startup.upgradeDialog.version";
+    const dialogReason = await (async () => {
+      if (!Services.prefs.getBoolPref("browser.proton.enabled", true)) {
+        return "no-proton";
+      }
+      if (!BrowserHandler.majorUpgrade) {
+        return "not-major";
+      }
+      const lastVersion = Services.prefs.getIntPref(dialogVersionPref, 0);
+      if (lastVersion > dialogVersion) {
+        return "newer-shown";
+      }
+      if (lastVersion === dialogVersion) {
+        return "already-shown";
+      }
+      if (
+        !Services.prefs.getBoolPref(
+          "browser.messaging-system.whatsNewPanel.enabled",
+          true
+        )
+      ) {
+        return "no-whatsNew";
+      }
+      if (!Services.prefs.getBoolPref("browser.aboutwelcome.enabled", true)) {
+        return "no-welcome";
+      }
+      if (!Services.policies.isAllowed("postUpdateCustomPage")) {
+        return "disallow-postUpdate";
       }
-      // If no experiment go ahead with default experience
-      if (willPrompt && !isFeatureEnabled) {
-        let win = BrowserWindowTracker.getTopWindow();
-        DefaultBrowserCheck.prompt(win);
-      }
-      // If in experiment notify ASRouter to dispatch message
-      if (isFeatureEnabled) {
-        ASRouter.waitForInitialized.then(() =>
-          ASRouter.sendTriggerMessage({
-            browser: BrowserWindowTracker.getTopWindow()?.gBrowser
-              .selectedBrowser,
-            // triggerId and triggerContext
-            id: "defaultBrowserCheck",
-            context: { willShowDefaultPrompt: willPrompt },
-          })
-        );
-      }
-    });
+
+      // Check enabled last to avoid waiting on remote data in the common case.
+      await NimbusFeatures.upgradeDialog.ready();
+      return NimbusFeatures.upgradeDialog.isEnabled() ? "" : "disabled";
+    })();
+
+    // Show the upgrade dialog if allowed and remember the version.
+    if (!dialogReason) {
+      Services.prefs.setIntPref(dialogVersionPref, dialogVersion);
+      this._showUpgradeDialog();
+      return;
+    }
+
+    // Determine if the modal prompt or infobar message should be shown.
+    const [willPrompt] = await Promise.all([
+      DefaultBrowserCheck.willCheckDefaultBrowser(/* isStartupCheck */ true),
+      ExperimentAPI.ready(),
+    ]);
+    let { DefaultBrowserNotification } = ChromeUtils.import(
+      "resource:///actors/AboutNewTabParent.jsm",
+      {}
+    );
+    let isFeatureEnabled = ExperimentAPI.getExperiment({
+      featureId: "infobar",
+    })?.branch.feature.enabled;
+    if (willPrompt) {
+      // Prevent the related notification from appearing and
+      // show the modal prompt.
+      DefaultBrowserNotification.notifyModalDisplayed();
+    }
+    // If no experiment go ahead with default experience
+    if (willPrompt && !isFeatureEnabled) {
+      let win = BrowserWindowTracker.getTopWindow();
+      DefaultBrowserCheck.prompt(win);
+    }
+    // If in experiment notify ASRouter to dispatch message
+    if (isFeatureEnabled) {
+      await ASRouter.waitForInitialized;
+      ASRouter.sendTriggerMessage({
+        browser: BrowserWindowTracker.getTopWindow()?.gBrowser.selectedBrowser,
+        // triggerId and triggerContext
+        id: "defaultBrowserCheck",
+        context: { willShowDefaultPrompt: willPrompt },
+      });
+    }
   },
 
   /**
    * Open preferences even if there are no open windows.
    */
   _openPreferences(...args) {
     let chromeWindow = BrowserWindowTracker.getTopWindow();
     if (chromeWindow) {

--- a/browser/components/nsIBrowserHandler.idl
+++ b/browser/components/nsIBrowserHandler.idl
@@ -7,15 +7,16 @@
 interface nsICommandLine;
 
 [scriptable, uuid(8D3F5A9D-118D-4548-A137-CF7718679069)]
 interface nsIBrowserHandler : nsISupports
 {
   attribute AUTF8String startPage;
   attribute AUTF8String defaultArgs;
   attribute boolean kiosk;
+  attribute boolean majorUpgrade;
 
   /**
    * Extract the width and height specified on the command line, if present.
    * @return A feature string with a prepended comma, e.g. ",width=500,height=400"
    */
   AUTF8String getFeatures(in nsICommandLine aCmdLine);
 };

--- a/browser/components/tests/browser/browser_browserGlue_upgradeDialog.js
+++ b/browser/components/tests/browser/browser_browserGlue_upgradeDialog.js
@@ -208,11 +208,47 @@ add_task(async function exit_early() {
 
   Assert.equal(
     mock.setAsDefault.callCount,
     0,
     "Only 1 screen to skip when default"
   );
 });
 
+add_task(async function not_major_upgrade() {
+  await BROWSER_GLUE._maybeShowDefaultBrowserPrompt();
+});
+
+add_task(async function remote_disabled() {
+  // TODO(bug 1701948): Set up actual remote defaults.
+  NimbusFeatures.upgradeDialog._onRemoteReady();
+  Services.prefs.setBoolPref("browser.startup.upgradeDialog.enabled", false);
+
+  // Simulate starting from a previous version.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.startup.homepage_override.mstone", "88.0"]],
+  });
+  Cc["@mozilla.org/browser/clh;1"].getService(Ci.nsIBrowserHandler).defaultArgs;
+
+  await BROWSER_GLUE._maybeShowDefaultBrowserPrompt();
+
+  Services.prefs.clearUserPref("browser.startup.upgradeDialog.enabled");
+});
+
+add_task(async function show_major_upgrade() {
+  const promise = waitForDialog();
+  await BROWSER_GLUE._maybeShowDefaultBrowserPrompt();
+  await promise;
+
+  Assert.ok(true, "Upgrade dialog opened and closed from major upgrade");
+});
+
+add_task(async function dont_reshow() {
+  await BROWSER_GLUE._maybeShowDefaultBrowserPrompt();
+});
+
 registerCleanupFunction(() => {
   getShellService.restore();
+  Cc["@mozilla.org/browser/clh;1"].getService(
+    Ci.nsIBrowserHandler
+  ).majorUpgrade = false;
+  Services.prefs.clearUserPref("browser.startup.upgradeDialog.version");
 });

--- a/browser/components/tests/browser/whats_new_page/browser.ini
+++ b/browser/components/tests/browser/whats_new_page/browser.ini
@@ -3,10 +3,11 @@ skip-if = verify
 reason = This is a startup test. Verify runs tests multiple times after startup.
 support-files =
   active-update.xml
   updates/0/update.status
 prefs =
   app.update.altUpdateDirPath='<test-root>/browser/components/tests/browser/whats_new_page'
   app.update.disabledForTesting=false
   browser.startup.homepage_override.mstone="60.0"
+  browser.startup.upgradeDialog.enabled=false
 
 [browser_whats_new_page.js]

--- a/toolkit/components/nimbus/ExperimentAPI.jsm
+++ b/toolkit/components/nimbus/ExperimentAPI.jsm
@@ -60,16 +60,20 @@ const MANIFEST = {
       prefsButtonIcon: {
         type: "string",
       },
     },
   },
   "password-autocomplete": {
     description: "A special autocomplete UI for password fields.",
   },
+  upgradeDialog: {
+    description: "The dialog shown for major upgrades",
+    enabledFallbackPref: "browser.startup.upgradeDialog.enabled",
+  },
 };
 
 function isBooleanValueDefined(value) {
   return typeof value === "boolean";
 }
 
 const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
 const { XPCOMUtils } = ChromeUtils.import(