The flatpak org.freedesktop.Platform.ffmpeg-full is necessary for many users. Without it, videos are lagging or not properly playing. For unexperienced users, this might be a reason to not use firefox. The many posts about video issues is surely only the tip of the iceberg.
Even with the knowledge about this issue, everytime firefox updates it's runtime environment, the user has to manually update ffmpeg-full. For a developer this might be ok, but this not ok for end users.
I don't know how to solve this, but right now we dont really have an option to auto-update ffmpeg-full, we even dont have the info about the change in firefox! I only realize it, when videos are not playing properly again.