diff -up firefox-43.0/firefox-43.0/browser/app/profile/firefox.js.addons firefox-43.0/firefox-43.0/browser/app/profile/firefox.js
--- firefox-43.0/browser/app/profile/firefox.js.addons	2015-12-10 10:18:32.942990945 +0100
+++ firefox-43.0/browser/app/profile/firefox.js	2015-12-10 10:22:36.435169388 +0100
@@ -71,7 +71,8 @@ pref("extensions.hotfix.certs.2.sha1Fing
 
 // Disable add-ons that are not installed by the user in all scopes by default.
 // See the SCOPE constants in AddonManager.jsm for values to use here.
-pref("extensions.autoDisableScopes", 15);
+pref("extensions.autoDisableScopes", 0);
+pref("extensions.showMismatchUI", false);
 
 // Require signed add-ons by default
 pref("xpinstall.signatures.required", true);
diff -up firefox-43.0/firefox-43.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm.addons firefox-43.0/firefox-43.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm
--- firefox-43.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm.addons	2015-12-08 19:06:45.000000000 +0100
+++ firefox-43.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm	2015-12-10 10:18:32.944990955 +0100
@@ -2523,11 +2523,7 @@ this.XPIProvider = {
 
       AddonManagerPrivate.markProviderSafe(this);
 
-      if (aAppChanged === undefined) {
-        // For new profiles we will never need to show the add-on selection UI
-        Services.prefs.setBoolPref(PREF_SHOWN_SELECTION_UI, true);
-      }
-      else if (aAppChanged && !this.allAppGlobal &&
+      if (aAppChanged && !this.allAppGlobal &&
                Preferences.get(PREF_EM_SHOW_MISMATCH_UI, true)) {
         if (!Preferences.get(PREF_SHOWN_SELECTION_UI, false)) {
           // Flip a flag to indicate that we interrupted startup with an interactive prompt
@@ -2547,6 +2543,9 @@ this.XPIProvider = {
             flushCaches = true;
           }
         }
+      } else {
+        // For new profiles we will never need to show the add-on selection UI
+        Services.prefs.setBoolPref(PREF_SHOWN_SELECTION_UI, true);
       }
 
       if (flushCaches) {