Don't use deprecated InstallTrigger for Firefox detection

Fixes #4033

Author: philc

lib/utils.js

@@ -20,7 +20,7 @@ if (window.browser && browser.runtime && browser.runtime.getBrowserInfo)
 var Utils = {
   isFirefox: (function() {
     // We want this browser check to also cover Firefox variants, like LibreWolf. See #3773.
-    const isFirefox = typeof InstallTrigger !== 'undefined';
+    const isFirefox = (typeof browser === "object" && browser.runtime.getURL("").startsWith("moz"));
     return () => isFirefox;
   })(),