The only real issue I have is that searching for communities I know exist on other instances often fails, and opening them in their home instances doesn't offer a subscribe button to my host instance.
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.