Enable storage.sync to fix broken webextensions

Cloned from mullvad-browser#250 (closed):

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.

[...]

#41424 (closed) was a bit draconian with extensions, since if Firefox Sync is disabled this is just another (smaller) local storage. Of course, extensions can live without it, but not all them are prepared to properly fallback.