BleepingComputer.com: Mozilla Firefox Back Button Not Working

I am having a small problem with my browser, Mozilla Firefox. When I click the little arrow next to the back button to see the previously viewed pages, all that shows up is a small square with nothing in it (this also happens with the forward button).

http://img399.imageshack.us/my.php?image=pic1229748304xp7.jpg
I recently downloaded and installed Comodo Antivirus, toolbar and firewall.