Says here the Cisco codec allows you to use H.264 in WebRTC with gstreamer and Firefox. It does not enable generic H.264 playback, only WebRTC.

https://github.com/cisco/openh264/issues/2835 https://fedoraproject.org/wiki/OpenH264

Because generic support for H264 in Firefox came in Firefox 43, four years ago. If Firefox finds ffmpeg, it uses it.

True but it’s a poor experience getting it to work. It breaks every few upgrades, poorly documented and mainly in the the bugtracker, and is unofficial hack because it’s not a licensed decoder.