Google is known to sabotage non-Chrome browsers and ad-blockers on YouTube, accidentally on purpose at every chance it can get. That's hardly Firefox's fault.
I use youtube a worrying amount, all through firefox (and I am also an ADHD 1000 tabs open person), and I have never had this experience across multiple computers.
Youtube freezes for me on Windows with Firefox. Sometimes it freezes the whole browser so added a rule to block it in uBo and use another browser for it (which also helps me reduce my time on youtube thanks to the added friction). I don't know if its an issue with my specific hardware or what. In Mac and Linux (different PC) it works fine.
You need to keep the same youtube tab open and navigate the website a considerable amount on it for it to occur. I use the same tabs repeatedly, and by the end of the day I get an incredibly sluggish site. Not the end of the world, since I just close the tab and start a new one, but it does exist.
Firefox has had documented issues with youtube for the better part of a decade at least.
