2730 + 2750: Storage API + Offline Cache

ESR52.x doesn't use the new site storage UI. FF61+ the issue is resolved, so let's enforce offline cache (2730) as false again

user.js

@@ -1390,8 +1390,8 @@ user_pref("network.cookie.leave-secure-alone", true); // default: true
  * [WARNING] This *will* break other extensions including legacy, and *will* break some sites ***/
    // user_pref("dom.indexedDB.enabled", false);
 /* 2730: disable offline cache
- * [NOTE] For FF60.0.1 and under, this is required 'true' for Storage API (2750) ***/
-   // user_pref("browser.cache.offline.enable", false);
+ * [NOTE] For FF51-FF60 (ESR not included), this is required 'true' for Storage API (2750) ***/
+user_pref("browser.cache.offline.enable", false);
 /* 2730b: disable offline cache on insecure sites (FF60+)
  * [1] https://blog.mozilla.org/security/2018/02/12/restricting-appcache-secure-contexts/ ***/
 user_pref("browser.cache.offline.insecure.enable", false);
@@ -1402,15 +1402,15 @@ user_pref("offline-apps.allow_by_default", false);
 /* 2740: disable service workers cache and cache storage
  * [1] https://w3c.github.io/ServiceWorker/#privacy ***/
 user_pref("dom.caches.enabled", false);
-/* 2750: disable Storage API
+/* 2750: disable Storage API (FF51+)
  * The API gives sites the ability to find out how much space they can use, how much
  * they are already using, and even control whether or not they need to be alerted
  * before the user agent disposes of site data in order to make room for other things.
- * [NOTE] For FF60.0.1 and under, if Storage API is enabled, then Offline Cache (2730) must be also be enabled
+ * [NOTE] For FF51-FF60 (ESR not included), if Storage API is enabled, then Offline Cache (2730) must be also be enabled
  * [1] https://developer.mozilla.org/docs/Web/API/StorageManager
  * [2] https://developer.mozilla.org/docs/Web/API/Storage_API
  * [3] https://blog.mozilla.org/l10n/2017/03/07/firefox-l10n-report-aurora-54/ ***/
-   // user_pref("dom.storageManager.enabled", false); // (FF51+)
+   // user_pref("dom.storageManager.enabled", false);
 
 /*** 2800: SHUTDOWN [SETUP]
      You should set the values to what suits you best. Be aware that the settings below clear