Bug 1670966 - Add a separate pref to enable Non-Fission window option, r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D93413

browser/base/content/browser.js

@@ -9518,6 +9518,14 @@ if (AppConstants.NIGHTLY_BUILD) {
         return;
       }
 
+      const openNonFissionWindowOption = Services.prefs.getBoolPref(
+        "fission.openNonFissionWindowOption",
+        false
+      );
+      if (!openNonFissionWindowOption) {
+        return;
+      }
+
       let newFissionWindow = document.getElementById("Tools:FissionWindow");
       let newNonFissionWindow = document.getElementById(
         "Tools:NonFissionWindow"

modules/libpref/init/StaticPrefList.yaml

@@ -3612,6 +3612,13 @@
   value: true
   mirror: always
 
+# If true, show option for opening a non-Fission window in a menu, when Fission
+# is enabled.
+- name: fission.openNonFissionWindowOption
+  type: bool
+  value: false
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "font."
 #---------------------------------------------------------------------------