I successfully setup sync (https://support.mozilla.org/en-US/kb/use-sync-share-bookmarks-and-more-with-firefox-mobile). I check Settings->Sync and waited until sync. has terminated. Finally I got no Bookmarks or password synced. I do not expect to get add-ons synced, but all other stuff should work. No "about:about:sync-log" for further analysis on Android available.