Hello,

First, please try clearing cookies and site data for YouTube and also running Firefox in the Troubleshoot Mode. Does it help?

No, safe mode and clearing cookies and site data for YouTube do not work, even after a complete reset of FireFox settings, Youtube does not work. I'm going to try reinstalling FireFox now, and then update this answer.

Changed: After completely reinstalling FireFox and then deleting files from the Appdata/Roaming and Appdata/Local folders, YouTube still does not work.

(Despite the fact that I live in Russia, and YouTube are blocked by the state, we use a special workaround called Zapret (open source code is available on GitHub).) Again, going back to the previous words, Youtube is working with him as in Google Chrome and Yandex browser, and I have only two reasons why

1. Using a different engine to launch the browser. It is possible that Youtube's work in Firefox is hindered either by the Quantum engine or by Gecko working with it, while Google Chrome and Yandex run on the Blink engine and use the V8 version, which is responsible for processing and executing JavaScript code.

2. There are problems with transferring data to FireFox. FireFox uses the HTTPS protocol to securely connect to websites, which is an extension of the HTTP protocol. In turn, Google Chrome and Yandex use HTTP/1.1, HTTP/2 and QUIC protocols to load web pages.