musescore works for me.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

You can toggle DRM off/on to see if that makes Firefox (re)load DRM components.

• Settings -> General: Digital Rights Management (DRM) Content
• https://support.mozilla.org/en-US/kb/enable-drm

You can possibly check the XHR requests in the Browser Console if this doesn't work.

• https://firefox-source-docs.mozilla.org/devtools-user/browser_console/

Okay well I fixed it... By reinstalling firefox *shrug*... and synching with my old profile still works. So absolutly no idea what went wrong there, I almost blame cosmic rays...

So.... if it helps anyone here my answers to the troubleshooting steps :

• I also noticed I had the exact same issue with Prime video too.
• DRM Content was on.
• There was no XHR Request on the console that I can see. (screenshot)
• Toggleing it on and off changed the behaviour somewhat : now instead of instantly having the error page, it seems like the video is loading (for like 1-2 minutes) then the error page appears.
• Also I have a new message on top of firefox saying "Firefox is installing component needed for the playback of audio or video on this page. Please try again later" (In french, screenshot).

This message lead me to other people having the same issue and mostly dead threads without any helps (french ones at least)

I seen one person in.. 2016... Suggesting a full reinstallation and I did it and it worked.

Musescore started working perfectly again after the fresh isntall.

So thanks you, no idia what went wrong there, but at least it's fixed !