jyavenard
There shouldn't be much difference for h264 videos between Safari and Firefox, except in Full Screen. For full screen, Safari uses the OS to render the images and can have a very efficient code path. Firefox doesn't support it at all (for now)
Firefox doesn't use a blacklist of drivers / GPU like it does on window to enable HW decoding or not. Typically mac hardware is far more reliable.
the media.hardware-video-decoding.force-enabled is only of use on Windows in the event a card is blacklisted and you want to override the blacklist
Equivalent features are provided in the Video Toolbox framework from OS X 10.8 and later.
Firefox only supports 10.9 and later from now on, and no longer uses VDA.
Unlike VDA, when you create a decoder, you just get one. You then query if it's a hardware decoder or not, but the behaviour is fundamentally the same between SW and HW.
For some reasons, the VT frame gives us a software decoder most of the time for some users.
Until now, I had only ever seen Mac Pro (including 2013 trashcan model) to not provide hardware decoding support.
If you attempt to decode a video that is less than 144 pixels high, you always get a software decoder.