hi, what results does https://www.youtube.com/html5 show?

this is what it shows

even in safe mode the same video will not play

thanks, do you happen to use a N version of windows (shipping without media codecs)? if so you might need to install the media feature pack offered by microsoft to be able to playback all formats: Fix video and audio playback problems on Firefox for Windows

in addition to that, please enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named media.wmf.vp9.enabled. double-click it and change its value to false - otherwise users of the media feature pack on windows 10 v1809 might observe frequent tab crashes currently...

i did the config bit but none of the downloads are for my windows

what's your exact version/edition of windows then? https://www.howtogeek.com/236195/how-to-find-out-which-build-and-version-of-windows-10-you-have/

but the 1802 download gives this message

• 1803

oh yap, then it's no N version and the article doesn't apply...

can you check that media.mp4.enabled is enabled in about:config or else try to refresh firefox? Refresh Firefox - reset add-ons and settings

still got the same problem

i guess i just cant use firefox anymore

i have had a windows error something about system32/mfperfhelper.dll not meant to run on windows or corrupted, and nothing i do helps so ill just assume they are connected

yes, unfortunately h.264 is a closed codec asking for royalties so firefox doesn't ship with it out of the box and would rely on the capabilities of the operating system or the graphics hardware to playback such videos.

unfortunately if you've already refreshed firefox, your windows is a normal edition (not N) and your graphics driver is pretty fresh out of the box i'm out of ideas what might go wrong here fairly quickly :-/