--- a/browser/components/urlbar/UrlbarInput.jsm
+++ b/browser/components/urlbar/UrlbarInput.jsm
@@ -14,16 +14,17 @@ XPCOMUtils.defineLazyModuleGetters(this,
   AppConstants: "resource://gre/modules/AppConstants.jsm",
   BrowserUsageTelemetry: "resource:///modules/BrowserUsageTelemetry.jsm",
   BrowserUtils: "resource://gre/modules/BrowserUtils.jsm",
   ExtensionSearchHandler: "resource://gre/modules/ExtensionSearchHandler.jsm",
   ObjectUtils: "resource://gre/modules/ObjectUtils.jsm",
   PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.jsm",
   ReaderMode: "resource://gre/modules/ReaderMode.jsm",
   PartnerLinkAttribution: "resource:///modules/PartnerLinkAttribution.jsm",
+  SearchUtils: "resource://gre/modules/SearchUtils.jsm",
   Services: "resource://gre/modules/Services.jsm",
   UrlbarController: "resource:///modules/UrlbarController.jsm",
   UrlbarEventBufferer: "resource:///modules/UrlbarEventBufferer.jsm",
   UrlbarPrefs: "resource:///modules/UrlbarPrefs.jsm",
   UrlbarProvidersManager: "resource:///modules/UrlbarProvidersManager.jsm",
   UrlbarQueryContext: "resource:///modules/UrlbarUtils.jsm",
   UrlbarTokenizer: "resource:///modules/UrlbarTokenizer.jsm",
   UrlbarUtils: "resource:///modules/UrlbarUtils.jsm",
@@ -104,17 +105,21 @@ class UrlbarInput {
     this._lastValidURLStr = "";
     this._valueOnLastSearch = "";
     this._resultForCurrentValue = null;
     this._suppressStartQuery = false;
     this._suppressPrimaryAdjustment = false;
     this._untrimmedValue = "";
     this._searchModesByBrowser = new WeakMap();
 
-    UrlbarPrefs.addObserver(this);
+    this.QueryInterface = ChromeUtils.generateQI([
+      "nsIObserver",
+      "nsISupportsWeakReference",
+    ]);
+    this._addObservers();
 
     // This exists only for tests.
     this._enableAutofillPlaceholder = true;
 
     // Forward certain methods and properties.
     const CONTAINER_METHODS = [
       "getAttribute",
       "hasAttribute",
@@ -1255,18 +1260,23 @@ class UrlbarInput {
    *   The name of the search engine to restrict to.
    * @param {UrlbarUtils.RESULT_SOURCE} source
    *   A result source to restrict to.
    * @param {string} entry
    *   How search mode was entered. This is recorded in event telemetry. One of
    *   the values in UrlbarUtils.SEARCH_MODE_ENTRY.
    */
   setSearchMode({ engineName, source, entry }) {
-    if (!UrlbarPrefs.get("update2")) {
-      // Exit search mode.
+    // Exit search mode if update2 is disabled or the passed-in engine is
+    // invalid or hidden.
+    let engine = Services.search.getEngineByName(engineName);
+    if (
+      !UrlbarPrefs.get("update2") ||
+      (engineName && (!engine || engine.hidden))
+    ) {
       engineName = null;
       source = null;
     }
 
     // As an optimization, bail if the given search mode is already active.
     // Otherwise browser_preferences_usage.js fails due to accessing the
     // browser.urlbar.placeholderName pref (via BrowserSearch.initPlaceHolder
     // below) too many times.  That test does not enter search mode, but it
@@ -1583,17 +1593,38 @@ class UrlbarInput {
     }
 
     this.setSearchMode(searchMode);
     this._setValue(result.payload.query?.trimStart() || "", false);
     this.startQuery({ allowAutofill: false });
     return true;
   }
 
+  observe(subject, topic, data) {
+    switch (topic) {
+      case SearchUtils.TOPIC_ENGINE_MODIFIED: {
+        switch (data) {
+          case SearchUtils.MODIFIED_TYPE.CHANGED:
+          case SearchUtils.MODIFIED_TYPE.REMOVED:
+            if (this.searchMode?.engineName == subject.name) {
+              // We exit search mode if the current search mode engine was removed.
+              this.setSearchMode(this.searchMode);
+            }
+            break;
+        }
+        break;
+      }
+    }
+  }
+
   // Private methods below.
+  _addObservers() {
+    UrlbarPrefs.addObserver(this);
+    Services.obs.addObserver(this, SearchUtils.TOPIC_ENGINE_MODIFIED, true);
+  }
 
   _getURIFixupInfo(searchString) {
     let flags =
       Ci.nsIURIFixup.FIXUP_FLAG_FIX_SCHEME_TYPOS |
       Ci.nsIURIFixup.FIXUP_FLAG_ALLOW_KEYWORD_LOOKUP;
     if (this.isPrivate) {
       flags |= Ci.nsIURIFixup.FIXUP_FLAG_PRIVATE_CONTEXT;
     }

--- a/browser/components/urlbar/tests/browser/browser.ini
+++ b/browser/components/urlbar/tests/browser/browser.ini
@@ -148,16 +148,17 @@ run-if = e10s
 [browser_searchMode_alias_replacement.js]
 support-files =
   searchSuggestionEngine.xml
   searchSuggestionEngine.sjs
 [browser_searchMode_autofill.js]
 [browser_searchMode_clickLink.js]
 support-files =
   dummy_page.html
+[browser_searchMode_engineRemoval.js]
 [browser_searchMode_no_results.js]
 [browser_searchMode_pickResult.js]
 [browser_searchMode_setURI.js]
 [browser_searchMode_suggestions.js]
 support-files =
   searchSuggestionEngine.xml
   searchSuggestionEngine.sjs
 [browser_searchMode_switchTabs.js]

new file mode 100644
--- /dev/null
+++ b/browser/components/urlbar/tests/browser/browser_searchMode_engineRemoval.js
@@ -0,0 +1,103 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests that we exit search mode when the search mode engine is removed.
+ */
+
+"use strict";
+
+// Tests that we exit search mode in the active tab when the search mode engine
+// is removed.
+add_task(async function activeTab() {
+  let engine = await Services.search.addEngineWithDetails("Test", {
+    template: "http://example.com/?search={searchTerms}",
+  });
+  await Services.search.moveEngine(engine, 0);
+
+  await UrlbarTestUtils.promiseAutocompleteResultPopup({
+    window,
+    value: "ex",
+  });
+  await UrlbarTestUtils.enterSearchMode(window);
+  // Sanity check: we are in the correct search mode.
+  await UrlbarTestUtils.assertSearchMode(window, {
+    engineName: engine.name,
+    entry: "oneoff",
+  });
+  await Services.search.removeEngine(engine);
+  // Check that we are no longer in search mode.
+  await UrlbarTestUtils.assertSearchMode(window, null);
+});
+
+// Tests that we exit search mode in a background tab when the search mode
+// engine is removed.
+add_task(async function backgroundTab() {
+  let engine = await Services.search.addEngineWithDetails("Test", {
+    template: "http://example.com/?search={searchTerms}",
+  });
+  await Services.search.moveEngine(engine, 0);
+
+  await UrlbarTestUtils.promiseAutocompleteResultPopup({
+    window,
+    value: "ex",
+  });
+  await UrlbarTestUtils.enterSearchMode(window);
+  let tab1 = gBrowser.selectedTab;
+  let tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+
+  // Sanity check: tab1 is still in search mode.
+  await BrowserTestUtils.switchTab(gBrowser, tab1);
+  await UrlbarTestUtils.assertSearchMode(window, {
+    engineName: engine.name,
+    entry: "oneoff",
+  });
+
+  // Switch back to tab2 so tab1 is in the background when the engine is
+  // removed.
+  await BrowserTestUtils.switchTab(gBrowser, tab2);
+  // tab2 shouldn't be in search mode.
+  await UrlbarTestUtils.assertSearchMode(window, null);
+  await Services.search.removeEngine(engine);
+
+  // tab1 should have exited search mode.
+  await BrowserTestUtils.switchTab(gBrowser, tab1);
+  await UrlbarTestUtils.assertSearchMode(window, null);
+  BrowserTestUtils.removeTab(tab2);
+});
+
+// Tests that we exit search mode in a background window when the search mode
+// engine is removed.
+add_task(async function backgroundWindow() {
+  let engine = await Services.search.addEngineWithDetails("Test", {
+    template: "http://example.com/?search={searchTerms}",
+  });
+  await Services.search.moveEngine(engine, 0);
+
+  let win1 = window;
+  await UrlbarTestUtils.promiseAutocompleteResultPopup({
+    window: win1,
+    value: "ex",
+  });
+  await UrlbarTestUtils.enterSearchMode(win1);
+  let win2 = await BrowserTestUtils.openNewBrowserWindow();
+
+  // Sanity check: win1 is still in search mode.
+  win1.focus();
+  await UrlbarTestUtils.assertSearchMode(win1, {
+    engineName: engine.name,
+    entry: "oneoff",
+  });
+
+  // Switch back to win2 so win1 is in the background when the engine is
+  // removed.
+  win2.focus();
+  // win2 shouldn't be in search mode.
+  await UrlbarTestUtils.assertSearchMode(win2, null);
+  await Services.search.removeEngine(engine);
+
+  // win1 should not be in search mode.
+  win1.focus();
+  await UrlbarTestUtils.assertSearchMode(win1, null);
+  await BrowserTestUtils.closeWindow(win2);
+});