I discovered the problem after I managed to break video playback while configuring the newly installed browser.
Firefox has an "advanced" setting (about:config only) to prevent media automatically playing which was preventing playback - even with the video player's buttons. about:config | media.autoplay.enabled | true -> breaks the player.
However there's another anti-tracking option that prevents embedded videos even being displayed - Preferences | Privacy & Security | Tracking Protection | click [Change Block List...] -> 'Basic' works, 'Strict' doesn't .
I have Firefox Nightly installed now (with no extensions yet) and a blank profile which works so there's no need. I do appreciate the offer to help though.
Firefox has an "advanced" setting (about:config only) to prevent media automatically playing which was preventing playback - even with the video player's buttons. about:config | media.autoplay.enabled | true -> breaks the player.
However there's another anti-tracking option that prevents embedded videos even being displayed - Preferences | Privacy & Security | Tracking Protection | click [Change Block List...] -> 'Basic' works, 'Strict' doesn't .