I had this issue when it was in dark mode. Especially on phone. PC did same thing on black mode on themes

By on your phone, you mean when you go to Youtube on firefox mobile?

Hi there! Mozilla is actively investigating if you want to follow along or add a comment here: https://bugzilla.mozilla.org/show_bug.cgi?id=1963511#c20

In the meantime, do you have Ublock Origin or any extensions installed that you can try turning off? If you try viewing in Private Browsing Mode, are you still running into the issue?

I've loaded in troubleshoot mode, same thing. Restarted firefox, same results, I even uninstalled and reinstalled both with saved options and fresh, same results. If I open in private window it loads but, I haven't signed in on a private window. I will look into the thread you posted

If anyone is still experiencing this issue, we believe the fastest way to get things working again is:

• Ensure your uBlock Origin filters are up to date (see the uBlock Origin Documentation for how to do this.
• Close all YouTube tabs
• In the Firefox hamburger menu go to Settings, then Privacy and Security and click on the Manage Data button. In the resulting dialog, search for youtube.com, select the row for site youtube.com and click "Remove Selected"
• Opening a new tab with youtube.com should now work.

If anyone follows these steps and still experiences the same symptoms, please let us know here.