Firefox za Android Support Forum

I've seen too many threads about this issue. This has been around for four years straight. No one ever managed to solve this problem and developers didn't care either. I've tried everything I've found so far: 1. Force sync on Android after sync on Desktop; 2. Disconnect and reconnect everything; 3. Uninstall and install again Firefox on both devices; 4. Clear data on both devices; 5. Login in different ways (e-mail, qr code, etc.); 6. Turn off one device, sync and turn on later; 7. Disconnect all other devices that isn't related with this problem; 8. Activate and deactivate 2 Factor login; Anyways, this sync feature doesn't seem to work at all. Almost all threads ended up abandoned, just like the app itself. Do anyone ever found a reliable solution for this?

Firefox for Android gets frozen when using it to open large PDF files. I have been meaning to change my default PDF viewer app, but once Firefox has been set as the default PDF viewer for Android, there will no way to change this default; as Firefox default's don't have the option to be deleted through Android's App Info menu.