diff --git a/toolkit/mozapps/handling/ContentDispatchChooser.sys.mjs b/toolkit/mozapps/handling/ContentDispatchChooser.sys.mjs
index 8228826dfb29..e916fdad4c4c 100644
--- a/toolkit/mozapps/handling/ContentDispatchChooser.sys.mjs
+++ b/toolkit/mozapps/handling/ContentDispatchChooser.sys.mjs
@@ -321,7 +321,8 @@ export class nsContentDispatchChooser {
 
   /**
    * Opens a dialog as a SubDialog on tab level.
-   * If we don't have a BrowsingContext we will fallback to a standalone window.
+   * If we don't have a BrowsingContext or tab level dialogs are not supported,
+   * we will fallback to a standalone window.
    * @param {string} aDialogURL - URL of the dialog to open.
    * @param {Object} aDialogArgs - Arguments passed to the dialog.
    * @param {BrowsingContext} [aBrowsingContext] - BrowsingContext associated
@@ -348,16 +349,20 @@ export class nsContentDispatchChooser {
         );
       }
 
-      let tabDialogBox = window.gBrowser.getTabDialogBox(topFrameElement);
-      return tabDialogBox.open(
-        aDialogURL,
-        {
-          features: resizable,
-          allowDuplicateDialogs: false,
-          keepOpenSameOriginNav: true,
-        },
-        aDialogArgs
-      ).closedPromise;
+      // If the app does not support window.gBrowser or getTabDialogBox(),
+      // fallback to the standalone application chooser window.
+      let getTabDialogBox = window.gBrowser?.getTabDialogBox;
+      if (getTabDialogBox) {
+        return getTabDialogBox(topFrameElement).open(
+          aDialogURL,
+          {
+            features: resizable,
+            allowDuplicateDialogs: false,
+            keepOpenSameOriginNav: true,
+          },
+          aDialogArgs
+        ).closedPromise;
+      }
     }
 
     // If we don't have a BrowsingContext, we need to show a standalone window.
diff --git a/uriloader/exthandler/tests/mochitest/browser.toml b/uriloader/exthandler/tests/mochitest/browser.toml
index 5c034060174a..2eaf6632ac83 100644
--- a/uriloader/exthandler/tests/mochitest/browser.toml
+++ b/uriloader/exthandler/tests/mochitest/browser.toml
@@ -145,6 +145,12 @@ support-files = [
   "file_txt_attachment_test.txt^headers^",
 ]
 
+["browser_standalone_application_chooser_window_fallback.js"]
+support-files = [
+  "FTPprotocolHandler.html",
+  "blank.html",
+]
+
 ["browser_txt_download_save_as.js"]
 support-files = [
   "file_txt_attachment_test.txt",
diff --git a/uriloader/exthandler/tests/mochitest/browser_standalone_application_chooser_window_fallback.js b/uriloader/exthandler/tests/mochitest/browser_standalone_application_chooser_window_fallback.js
new file mode 100644
index 000000000000..40496fb3b2cf
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_standalone_application_chooser_window_fallback.js
@@ -0,0 +1,67 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the fallback fixed in bug 1875460, if the modern tab dialog box is not
+// supported.
+
+const TEST_URL =
+  "https://example.com/browser/" +
+  "uriloader/exthandler/tests/mochitest/FTPprotocolHandler.html";
+
+add_task(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.external_protocol_requires_permission", false]],
+  });
+
+  // Load a page with an FTP link.
+  let browser = gBrowser.selectedBrowser;
+  BrowserTestUtils.startLoadingURIString(browser, TEST_URL);
+  await BrowserTestUtils.browserLoaded(browser, false, TEST_URL);
+
+  // Make sure no handler is set, forcing the dialog to show.
+  let protoSvc = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  let protoInfo = protoSvc.getProtocolHandlerInfo("ftp");
+  ok(!protoInfo.preferredApplicationHandler, "no preferred handler is set");
+  let handlers = protoInfo.possibleApplicationHandlers;
+  is(0, handlers.length, "no handler registered for ftp");
+  protoInfo.alwaysAskBeforeHandling = true;
+  let handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  handlerSvc.store(protoInfo);
+
+  // Delete getTabDialogBox from gBrowser, to test the fallback to the standalone
+  // application chooser window.
+  let _getTabDialogBox = gBrowser.getTabDialogBox;
+  delete gBrowser.getTabDialogBox;
+
+  let appChooserDialogOpenPromise = BrowserTestUtils.domWindowOpened(
+    null,
+    async win => {
+      await BrowserTestUtils.waitForEvent(win, "load");
+      Assert.ok(
+        win.document.documentURI ==
+          "chrome://mozapps/content/handling/appChooser.xhtml",
+        "application chooser dialog opened"
+      );
+      return true;
+    }
+  );
+  let link = "#link";
+  await BrowserTestUtils.synthesizeMouseAtCenter(link, {}, browser);
+  let appChooserDialog = await appChooserDialogOpenPromise;
+
+  let appChooserDialogClosePromise =
+    BrowserTestUtils.domWindowClosed(appChooserDialog);
+  let dialog = appChooserDialog.document.getElementsByTagName("dialog")[0];
+  let cancelButton = dialog.getButton("cancel");
+  cancelButton.click();
+  await appChooserDialogClosePromise;
+
+  // Restore the original getTabDialogBox(), to not affect other tests.
+  gBrowser.getTabDialogBox = _getTabDialogBox;
+});