Apparently, crashing YouTube is no longer compatible with Windows 8. Today I had to restart my computer five times because YouTube freezing and crashing managed to break my entire desktop. This only happens with Firefox. Also, YouTube is much slower to load on Firefox, which will most likely make Windows 8 crash more often.
I understand that there is some crashing on Windows 8 using Flash on youtube. Please see if it continues after disabling hardware accelerations. Do you also have crash ids of the times that it did crash? This will help with investigation. Copy the last 5 reports listed on about:crashes page with the bp- in front of the id.
Some problems with Flash video playback can be resolved by disabling hardware acceleration in your Flash Player settings. (See this article for more information on using the Flash plugin in Firefox).