0505: system add-on updates, fixes #172

Note: I am not 100% sure what happens with an app update. If this is divorced from that check now, you should be able to get FF updated without any system addons. We'll have to wait until 62 needs an update to test it. In the meantime I've edited the [NOTE]. I've also left this inactive (eg imagine if they pushed a critical update for formfill), so this is an end-user decision. Added to sticky to revisit this pref

user.js

@@ -352,8 +352,9 @@ user_pref("network.allow-experiments", false);
 user_pref("app.normandy.enabled", false);
 user_pref("app.normandy.api_url", "");
 user_pref("app.shield.optoutstudies.enabled", false);
-/* 0505: block URL used for System Add-on updates (FF44+)
- * [NOTE] You will not get any System Add-on updates except when you update Firefox ***/
+/* 0505: disable System Add-on updates
+ * [NOTE] In FF61 and lower, you will not get any System Add-on updates except when you update Firefox ***/
+   // user_pref("extensions.systemAddon.update.enabled", false); // (FF62+)
    // user_pref("extensions.systemAddon.update.url", "");
 /* 0506: disable PingCentre telemetry (used in several System Add-ons) (FF57+)
  * Currently blocked by 'datareporting.healthreport.uploadEnabled' (see 0333) ***/