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 what is talking about. Tor already had vulnerabilities which worked through Javascript found on websites, so many people keep it disabled.