Sounds similar to this issue. Can you try the same step I suggested there (use troubleshoot mode and try to replicate the issue) and report back?

I did start with troubleshoot mode, but firefox was freezing again with youtube. Out of sheer curiosity I checked my GData Antivirus and saw that "cache2/entries" was in quarantine - see attached image. Could that be the problem? And why is a firefox part even in quarantine?

MalDenken, similar problems started for me a few months ago.

A workaround in my case is to not open any Youtube videos directly in the current tab.

If opening from either a bookmark or a website link, I use the "Open Link in New Tab" option instead.

I also have the "When you open a link, image or media in a new tab, switch to it immediately" option in settings unchecked, if that makes any difference.

So if nothing else works, then maybe you could try a day or two of only opening Youtube video links in a new tab then manually switching to them afterwards, and see if this helps?

Note, as said in my case the crashes only seemed to happen when opening a new link in the current tab. Whereas letting the next video in a playlist start after the currently playing one has finished has been fine.

Thank you all for your responses. I have created a new profile and started from scratch (yikes ... all those settings and passwords...). But I am watching since hours, uninterrupted, videos on youtube!

This is an annoying problem. Is there an older version of Firefox that I can download?