Firefox Flatpak requires ffmpeg-full 19.08 to fix choppy playback. It has to be that version (20.08 didn’t work for me). It doesn’t come out of the box and is essential for many popular websites that use h264. In order to fix this:
flatpak install org.freedesktop.Platform.ffmpeg-full/x86_64/19.08
ffmpeg-full version must match app runtime version. Firefox uses 19.08 runtime so only ffmpeg-full 19.08 will work for it.
Where can we check what runtime does an app use?
So if Firefox changes to ffmpeg 20.08, we need to manualy install it and delete 19.08?
You can check with flatpak remote-info flathub <appid>. AFAIK when ff updates the runtime then you will have to install ffmpeg-full again and remove the old one.