With March 20th update, I am no longer able to log in into Youtube, as soon as I log in, I get logged out. I was using the Nightly version until last week approximately and switched to the live version because I was experimenting this very same issue. I guess that whatever caused this issue, has been rolled in into the public. I tried disabling all my addons with no success. I also had youtube cookies to not be stored for security reasons, but allowing them doesn’t work either.
I can reproduce the issue all the time, I mean, it doesn’t work at all. I access Youtube, log in, and as soon as the page loads in dark theme, it reloads with me logged out.
I was able to get it working by nuking all settings, cookies, everything. Then I used sync to get most of my settings and stuff back. I lost cookies and my NoScript allowed sites, but at least it’s working.