Have cleared your browser cache? https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox

What happens when you try www.youtube.com?

1 - It's happened 2 days ago, probably when Firefox was automatically updated. I looked on the link provided and other Internet's suggestions but couldn't find a solution; 2 - When I select the www.youtube.com in the Chrome tab of Firefox it's hanging.

So you cleared your cache and still have the same issue?

I just reinstalled Firefox and there are following steps, which I made to work sites properly but not all of them: 1 - After I reinstall Firefox I had an error of unknown certificate issuer. I fixed it by enabling parameter in the about:config. But still don't know where to obtain the proper certificate; 2 - Then to make youtube working I tuned off the enhanced tracking protection; 3 - So, it seems, that something is not good for the Firefox version 91.