Since the FSF does not consider GPLv2 to be Apache 2.0 compatible, telegram-FOSS is GPLv2, and Android libraries are usually Apache 2.0, you have always had a problem.
They also use a ton of Apache2 libraries (at least 5 or 6) directly in their source repo.
They don't appear to even bother to properly give attribution/etc for the used software, so yeah.
This is so damn true I am LMAO at the other comments. So basically the entire community can get around the problem by releasing an Apache2 lic'd library that wraps whatever it is they need from firebase.
We'd be at a place no different from any other "FOSS" App on android today.
They also use a ton of Apache2 libraries (at least 5 or 6) directly in their source repo.
They don't appear to even bother to properly give attribution/etc for the used software, so yeah.