I have a guess: by default, Firefox has WebM VP9 disabled. You would see that as a red box on YouTube's diagnostic page here: https://www.youtube.com/html5 I've always assumed that's because the feature, while present in Firefox's code for a couple versions now, is not fully debugged. But if you want to give it a shot, here's where you enable it: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (2) In the search box above the list, type or paste '''webm''' and pause while the list is filtered (3) Double-click the '''media.mediasource.webm.enabled''' preference to switch it from false to true Then reload the YouTube test page to see whether it is detected (it should be). On pages of previously viewed videos, you might also need to reload to update the detection. Can you get the higher resolutions? (I turned it on, too, so I'll try to remember that if I start crashing.)

Yes, that worked! Cheers, mate.