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

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 and from what I've understand it's got something to do with the graphic card drivers and it's affecting both nVidia and ATI. If that is the case for you also, you may want to read on.