So I was able to get on my Windows 7 Home laptop and did some research for you. Youtube worked perfectly fine for me on Firefox version 97.
So I'm going to have you clear your cache, cookies, recent logins, everything. But before you do that, if there are any websites you are logged into but don't know your password, try to get those passwords, or just do a password reset.
If the browser is having a problem, I've been learning that by clearing your cache, cookies, offline data, recent logins, it's like taking a huge eraser to a chalkboard and making it a new, blank slate from which to work from.
So open Firefox, at the top you should see a toolbar with File, Edit, View, History, left click History. If you don't see this toolbar, hold down left ALT and press the letter F. Move your mouse over the word View and then over Toolbar and left click on Menu Bar. Once you left click History, left click on Clear Recent History. Set the Time Range to clear to: Everything. There are a total of 7 boxes you can click, click all 7 boxes so they all have a checkmark, and left click Ok.
This is to ensure that everything is cleared, cookies, cache, etc. I know, it sucks having to do that, but if something isn't working right, we have to just bite the bullet, and clear everything. Once everything has been cleared, close Firefox and wait a minute.
After a minute has passed, open Firefox and in the address bar type in or copy and paste: https://youtube.com
If the website loaded for you, don't log in, but try to get a video to play. If Malwarebytes is still blocking youtube, you can add it to the exception list in Malwarebytes.
