Enable storage.sync to fix broken webextensions

Currently, along with disabling Firefox Sync, we disable storage sync by setting webextensions.storage.sync.enabled to false. This created issues for some webextensions relying on sync storage, others would still partially work.

Since the update to MB 13.0, extensions using storage sync will have their popup menu load correctly but not reactive to any user input. Turning webextensions.storage.sync.enabled to true fixes those issues.

Could we permanently set this to true?