Hi,
Sadly, this is a very very common glitch (problem) with the Firefox web browser & it used to happen literally all the darn time in some past versions of the browser. Mozilla have improved the way the browser handles memory but this problem still persists, so your not alone in this experience by any stretch of the imagination.
**As has already been said, you just need to kill the process firefox.exe this then has the effect of releasing the grip the program has on your machines working memory set. You should NOT need to have to resort to actually re-starting the machine completely, you will have to reload Firefox though & there is no guarentee the same problem might not re-occur in future either. It seems to be one of those problems which has never fully been fixed by Mozilla & firefox.
Ivan