By default, Firefox (or Swiftfox) crashes when trying to view a full-screen video on say… YouTube. At least for me it used to crash until I found a fix. If that is the case for you also, you may want to read on.

Okay this has been a problem since the release of 3.5 (32 bit) so take a look folks, Baldy

How To Fix Full-Screen Flash Videos in Linux & Firefox (or Swiftfox)