It’s implemented by GMP (Gecko Media Plugin) API so the OpenH264 is not used through ffmpeg library but Firefox sandboxed interface, the same as Firefox uses for Widevine CDM plugin.
The OpenH264 GMP video playback is a fallback solution when system ffmpeg is missing and internal ffvpx library can’t decode the stream, so ffmpeg from RPM Fusion is always a better alternative if you can install it.
The video streams are decoded by system wide OpenH264 2.1.1 which is shipped by Fedora as mozilla-openh264 rpm package. Even if Mozilla OpenH264 (1.8.1) plugin is installed in your profile and claimed at about:plugins page, the Fedora system one is used.
OpenH264 plugin utilization, version and playback parameters can be confirmed from GMP log, just run Firefox on terminal as:
and look for OpenH264 version / path and so on.