If you have add-ons installed, have you tried disabling the add-ons? One of them might be interfering.
Also, some web pages (like photo galleries in some newspaper web pages) use this weird advertising network that makes it hard to use the back button. You have to hit the button 5 or 6 times before it actually goes back to a previous page.
So also try this: Go to google.com. In Firefox's address field manually type in "gatorzone.com". When that page loads, try the back button. Does it work?