Have you tried Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

I haven't tried that yet. It usually takes a few hours before the problem appears and it's intermittent. It might take a while to see if it appears in Troubleshoot Mode.

Also, I'm not sure this is related, but after some time I also notice that it starts to become difficult to click on URL links. As with the other issue, restarting the browser clears it up. I believe both of these issues began in v133.

If Windows Pro version, try testing in Windows Sandbox (VM) to see if you have the same issue. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/ You can also boot windows to safe mode with networking to test.

Hi John, I've seen several threads over on Reddit about YouTube issues in Firefox 133. It may be related to higher-than-HD video (like 1440p), but because the slowdown comes on gradually, it is very difficult to backtrack what may have caused the problem. One user noted that opening videos in a new tab (you can right-click (Ctrl+click) > Open Link in New Tab, or Command+click the link) improved performance, especially if you closed the source tab.