When you visit youtube on firefox 52, it sends you to the outdated browser page. Even if you click "remind me later", it will send you back to the same page.
Youtube still functions fully on this browser from early 2017 [besides a few ui glitches], but to get youtube to allow you in you need to go to about:config, right click, and make a new string, call it general.useragent.override and put anything as the value. [i personally use "fuckyou"]
This stops youtube's useragent check from sending you to the outdated browser page. Another way is to for example use the link on my homepage [when you use an href it skips the useragent check], but that only works until something triggers the check
my channel
Youtube
This link forces the mobile site [usefull for ie11 for example]
This link resets to use the desktop app