diff -up firefox-21.0/mozilla-release/browser/app/profile/firefox.js.addons firefox-21.0/mozilla-release/browser/app/profile/firefox.js
--- firefox-21.0/mozilla-release/browser/app/profile/firefox.js.addons	2013-05-11 21:19:20.000000000 +0200
+++ firefox-21.0/mozilla-release/browser/app/profile/firefox.js	2013-05-16 12:02:13.840660585 +0200
@@ -61,7 +61,8 @@ pref("extensions.hotfix.certs.1.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);
 
 // Dictionary download preference
 pref("browser.dictionaries.download.url", "https://addons.mozilla.org/%LOCALE%/firefox/dictionaries/");
diff -up firefox-21.0/mozilla-release/toolkit/mozapps/extensions/XPIProvider.jsm.addons firefox-21.0/mozilla-release/toolkit/mozapps/extensions/XPIProvider.jsm
diff -up firefox-15.0/mozilla-beta/toolkit/mozapps/extensions/XPIProvider.jsm.addons firefox-15.0/mozilla-beta/toolkit/mozapps/extensions/XPIProvider.jsm
--- firefox-15.0/mozilla-beta/toolkit/mozapps/extensions/XPIProvider.jsm.addons	2012-08-15 07:48:32.000000000 +0200
+++ firefox-15.0/mozilla-beta/toolkit/mozapps/extensions/XPIProvider.jsm	2012-08-21 11:56:41.441980035 +0200
@@ -1630,8 +1630,8 @@ var XPIProvider = {
       this.showUpgradeUI();
       flushCaches = true;
     }
-    else if (aAppChanged === undefined) {
-      // For new profiles we will never need to show the add-on selection UI
+    else {
+      // For all profiles we will never need to show the add-on selection UI
       Services.prefs.setBoolPref(PREF_SHOWN_SELECTION_UI, true);
     }