If you go to the home page (https://www.youtube.com/), does Firefox display an error about not being able to find the server, or some other error, or a blank page, etc.? Do you use a VPN to connect to YouTube through a different country? You can experiment with different DNS resolvers using the DNS over HTTPS settings. The following article has more information on that: [[Configure DNS over HTTPS protection levels in Firefox]] (I think the setting is below the ones shown in this article).