For example right now I’m facing an issue with Firefox (obtained from source registry.fedoraproject.org) on Fedora 31 SB.
My guess is the problem might be related not to Mozilla but to the integration into the OS.
(Problem: Firefox does not adopt the system’s default language for the UI. The language stays some “default” language.)
The localization issues should be fixed it the beta from Flathub:
The flatpaks from Fedora Registry are built from rpms and I guess it will take some time to fix the localization issues and add h264 using GStreamer instead of FFmpeg.