First thing I recommend for an issue like this is simply to try [[Troubleshoot Firefox issues using Safe Mode|running Firefox Safe Mode]]. This will help to rule out an issue caused by an extension or [[Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems#w_turn-off-hardware-acceleration|hardware acceleration]]. Next, although it may sound obvious, you should check to make sure that [[What to do if Firefox won't play any sounds|Firefox is not muted on Windows]]. If that doesn't help, the next step is to [[Fix video and audio playback problems on Firefox for Windows|reinstall the Windows Media Feature Pack]], which Firefox needs in order to play some HTML5 video formats. It's not uncommon for this pack to get damaged during a Windows update. Finally, [[Allow or block media autoplay in Firefox|toggling the autoplay blocking feature off]] helps in some cases. If none of that helps, navigate to the [https://www.youtube.com/html5 YouTube HTML5 test page] and let us know if any of the formats listed are red. They should all be blue, with the exception of ''MSE & WebM VP9'' which is sometimes red on certain systems and can usually just be disregarded. Hope this helps.