@roytam1's browsers (at least, the UXP-based ones) do play HTML5 videos OOTB; but I'd still say they're aimed at the MSFN crowd, not the general public. For one thing, they lack installers; for another, they aren't release builds.
We just had a discussion about that a few pages back. It seems you need to change your user agent. @msfntor suggested a SSUAO:
For a more general solution, you could just set general.useragent.override (without adding a specific site), or in Serpent, just make sure general.useragent.compatMode.firefox and general.useragent.compatMode.gecko are both set to true and general.useragent.compatMode.version is set to the Firefox version you want to pretend to be (e.g., "52.9", "55.0", "60.9", etc.).
In NM 28 there's a Firefox compatibility setting in Tools / Preferences that does the same thing.
Evidently many sites need to know you're using a Firefox fork in order to send you video with the right audio codec.