Firefox ships their own decoder now, Cisco donated a license of the MPEG patents to Mozilla.
I have been told that's only used for webrtc. playing <video> tags uses ffmpeg on linux or media foundation on windows.
Re:High quality GIF with FFmpeg
by le_lenny_face on Tuesday March 17, 2015 @10:29AM
(#9218140)
>playing <video> tags uses ffmpeg on linux
If only. It uses that broken pile of abstraction called GStreamer.
Afaik firefox uses system-provided h.264 decoders to support that since they don't want to ship one with firefox.