Recent crashes of certain multimedia contents (this includes Youtube videos, certain flash games and other applications) in conjunction with Firefox are most probably caused by a recent Flash 11.3 update and/or a malfunctioning Real Player browser plugin.

In order to remedy the problem, please perform the steps mentioned in these Knowledge Base articles:

Flash Plugin - Keep it up to date and troubleshoot problems

Adobe Flash plugin has crashed - Prevent it from happening again

Flash 11.3 doesn't load video in Firefox

Other, more technical information about these issues can be found under these Links:

http://forums.adobe.com/thread/1018071?tstart=0

http://blogs.adobe.com/asset/2012/06/inside-flash-player-protected-mode-for-firefox.html

Please tell us if this helped!