Fennec is even better than Firefox, it’s the same source but recompiled to allow all add-ons in the main app. So basically a stable Firefox nightly, if you wish.
It’s maintained by the folks behind the f-droid app store themselves, so arguably a highly trusted source.
Sync to Firefox desktop fully implemented and working.
You need to hold Ctrl + click on the languages you want.
However new posts are set to “undefined” by default and most peoplenever change it, so the whole feature seems a bit useless for now.
I guess once we have significantly large and diversified communities, it will make more sense.