Right next to the URL bar (right side), you can see the icon of a shield. If you click that and select "Advanced Security Settings", you go to a page where you can set up your "Security Level" (you can also just go to about:preferences#privacy). They have "Standard" (default), "Safer" and "Safest". The later disables Javascript on all websites, and your mixer, apparently, requires it, so it doesn't work on that level.
That's probably what he is talking about. Tor already had vulnerabilities which worked through Javascript found on websites, so many people keep it disabled.
You're right. Thank you for answering. My English is not good, so the expression is not in place!
Ok, that's something we do not handle differently than any other site requiring JavaScript.
You should at least see the fallback message indicating the problem very clearly: "Please enable JavaScript...".
This is only solvable by enabling JavaScript at least for the FoxMixer domain.
Anyway, if you want to avoid JavaScript and using the browser interface entirely, we can recommend the excellent open source python CLI for FoxMixer written by sega01.
https://github.com/teran-mckinney/foxmixer-python