Firefox has supported Html5 players (for video, audio and games) for easily over a decade (if not since oh 2008) as Mozilla even had a game called BrowserQuest back in 2012 to demonstrate that you can play games by Html5 and not need the Flash Player Plugin from Adobe.

https://support.mozilla.org/en-US/kb/end-support-adobe-flash Firefox 84 was the final (desktop) version to support Flash. Firefox version 85 (released on January 26, 2021) shipped without Flash support,

Major sites like Youtube and Twitch has been using Html5 players for perhaps a decade or so now and do not make use the Flash Player Plugin from Adobe anymore.

This makes sites more cross platform as the Flash Plugin was not on iOS/iPadOS and Adobe dropped Flash support for Android back in 2012 if I recall and Firefox 56.0 for Android finally dropped Flash support.

Short answer is Yes as what do you think news sites for example are using for their videos since the Flash Player is no longer used.

https://html5test.com/

Oh that reminds me, Youtube used to have a simple Html5 test page at https://www.youtube.com/html5 until several years ago. It was a simple way to check, especially for Windows KN or N users who needed media feature pack installed for the codecs and for Linux users who need packages like FFmpeg to have the codecs to see if everything is then supported.

jscher2000 here made his own copy of the old youtube test page at https://www.jeffersonscher.com/sumo/html5-video.html as the expected result should be "supported, probably, probably, supported, supported, supported".