If your in incognito nothing can be saved you should check with the site on saving your sessions or activity to reload again. One should never use incognito for anything other then using computer that isn't their own.

If Firefox becomes completely nonresponsive, that's a serious problem and certainly makes it difficult to recover.

If Firefox is responsive but sluggish, it might still be possible to find the tab playing audio so you can stop or close it. If you can find an audio icon on the player tab, that is the most direct method, however, you could also try Firefox's built-in Task Manager, which you can open using Shift+Esc. You can use Ctrl+F Find in Page in Firefox's Task Manager and then kill the tab on the media site if it is using too many resources. Be careful not to kill the main Firefox program tab, though, if you don't want to crash Firefox.