Backed out changeset 0e55b426edf3 (bug 1408867) for causing test failures. a=backout

mail/app/profile/all-thunderbird.js

@@ -375,10 +375,6 @@ pref("offline.download.download_messages",  0);
 // the network connection.
 pref("offline.autoDetect", true);
 
-// Disable preconnect and friends due to privacy concerns. They are not
-// sent through content policies.
-pref("network.http.speculative-parallel-limit", 0);
-
 // Expose only select protocol handlers. All others should go
 // through the external protocol handler route.
 // If you are changing this list, you may need to also consider changing the