diff --git a/docshell/base/URIFixup.jsm b/docshell/base/URIFixup.jsm
index 001a43bf0964d..8196c60e373db 100644
--- a/docshell/base/URIFixup.jsm
+++ b/docshell/base/URIFixup.jsm
@@ -55,6 +55,13 @@ XPCOMUtils.defineLazyServiceGetter(
   "nsIDNSService"
 );
 
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "handlerService",
+  "@mozilla.org/uriloader/handler-service;1",
+  "nsIHandlerService"
+);
+
 XPCOMUtils.defineLazyPreferenceGetter(
   lazy,
   "fixupSchemeTypos",
@@ -259,7 +266,15 @@ XPCOMUtils.defineLazyGetter(lazy, "knownSuffixes", () => {
   return suffixes;
 });
 
-function URIFixup() {}
+function URIFixup() {
+  // There are cases that nsIExternalProtocolService.externalProtocolHandlerExists() does
+  // not work well and returns always true due to flatpak. In this case, in order to
+  // fallback to nsIHandlerService.exits(), we test whether can trust
+  // nsIExternalProtocolService here.
+  this._trustExternalProtocolService = !lazy.externalProtocolService.externalProtocolHandlerExists(
+    `__dummy${Date.now()}__`
+  );
+}
 
 URIFixup.prototype = {
   get FIXUP_FLAG_NONE() {
@@ -329,7 +344,7 @@ URIFixup.prototype = {
       scheme &&
       (isCommonProtocol ||
         Services.io.getProtocolHandler(scheme) != lazy.defaultProtocolHandler ||
-        lazy.externalProtocolService.externalProtocolHandlerExists(scheme));
+        this._isKnownExternalProtocol(scheme));
 
     if (
       canHandleProtocol ||
@@ -631,6 +646,14 @@ URIFixup.prototype = {
 
   isDomainKnown,
 
+  _isKnownExternalProtocol(scheme) {
+    return this._trustExternalProtocolService
+      ? lazy.externalProtocolService.externalProtocolHandlerExists(scheme)
+      : lazy.handlerService.exists(
+          lazy.externalProtocolService.getProtocolHandlerInfo(scheme)
+        );
+  },
+
   classID: Components.ID("{c6cf88b7-452e-47eb-bdc9-86e3561648ef}"),
   QueryInterface: ChromeUtils.generateQI(["nsIURIFixup"]),
 };
diff --git a/docshell/test/unit/test_URIFixup_external_protocol_fallback.js b/docshell/test/unit/test_URIFixup_external_protocol_fallback.js
new file mode 100644
index 0000000000000..61fa921cc8814
--- /dev/null
+++ b/docshell/test/unit/test_URIFixup_external_protocol_fallback.js
@@ -0,0 +1,100 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+// Test whether fallback mechanism is working if don't trust nsIExternalProtocolService.
+
+const { MockRegistrar } = ChromeUtils.import(
+  "resource://testing-common/MockRegistrar.jsm"
+);
+
+add_task(async function setup() {
+  info(
+    "Prepare mock nsIExternalProtocolService whose externalProtocolHandlerExists returns always true"
+  );
+  const externalProtocolService = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  const mockId = MockRegistrar.register(
+    "@mozilla.org/uriloader/external-protocol-service;1",
+    {
+      getProtocolHandlerInfo: scheme =>
+        externalProtocolService.getProtocolHandlerInfo(scheme),
+      externalProtocolHandlerExists: () => true,
+      QueryInterface: ChromeUtils.generateQI(["nsIExternalProtocolService"]),
+    }
+  );
+  const mockExternalProtocolService = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  Assert.ok(
+    mockExternalProtocolService.externalProtocolHandlerExists("__invalid__"),
+    "Mock service is working"
+  );
+
+  info("Register new dummy protocol");
+  const dummyProtocolHandlerInfo = externalProtocolService.getProtocolHandlerInfo(
+    "dummy"
+  );
+  const handlerService = Cc[
+    "@mozilla.org/uriloader/handler-service;1"
+  ].getService(Ci.nsIHandlerService);
+  handlerService.store(dummyProtocolHandlerInfo);
+
+  info("Prepare test search engine");
+  await setupSearchService();
+  await addTestEngines();
+  await Services.search.setDefault(
+    Services.search.getEngineByName(kSearchEngineID)
+  );
+
+  registerCleanupFunction(() => {
+    handlerService.remove(dummyProtocolHandlerInfo);
+    MockRegistrar.unregister(mockId);
+  });
+});
+
+add_task(function basic() {
+  const testData = [
+    {
+      input: "mailto:test@example.com",
+      expected: isSupportedInHandlerService("mailto")
+        ? "mailto:test@example.com"
+        : "http://mailto:test@example.com/",
+    },
+    {
+      input: "keyword:search",
+      expected: "https://www.example.org/?search=keyword%3Asearch",
+    },
+    {
+      input: "dummy:protocol",
+      expected: "dummy:protocol",
+    },
+  ];
+
+  for (const { input, expected } of testData) {
+    assertFixup(input, expected);
+  }
+});
+
+function assertFixup(input, expected) {
+  const { preferredURI } = Services.uriFixup.getFixupURIInfo(
+    input,
+    Services.uriFixup.FIXUP_FLAG_FIX_SCHEME_TYPOS
+  );
+  Assert.equal(preferredURI.spec, expected);
+}
+
+function isSupportedInHandlerService(scheme) {
+  const externalProtocolService = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  const handlerService = Cc[
+    "@mozilla.org/uriloader/handler-service;1"
+  ].getService(Ci.nsIHandlerService);
+  return handlerService.exists(
+    externalProtocolService.getProtocolHandlerInfo(scheme)
+  );
+}
diff --git a/docshell/test/unit/xpcshell.ini b/docshell/test/unit/xpcshell.ini
index 21cf5f321ddea..21e63c2228d01 100644
--- a/docshell/test/unit/xpcshell.ini
+++ b/docshell/test/unit/xpcshell.ini
@@ -10,6 +10,7 @@ support-files =
 [test_browsing_context_structured_clone.js]
 [test_URIFixup.js]
 [test_URIFixup_check_host.js]
+[test_URIFixup_external_protocol_fallback.js]
 [test_URIFixup_forced.js]
 # Disabled for 1563343 -- URI fixup should be done at the app level in GV.
 skip-if = os == 'android'
@@ -24,6 +25,6 @@ skip-if =
 skip-if = true
 [test_privacy_transition.js]
 [test_subframe_stop_after_parent_error.js]
-skip-if = 
+skip-if =
   os == 'android'
   appname == 'thunderbird' # Needs to run without E10s, can't do that.