There are several problems with the new firefox release 1.5x and vB boards. Hopefully FF will fix them in the future.
What you might want to try doing is deleting both cache and cookies in FF, and then shutting down firefox and restarting it. Shut down FF from the task manager. Sometimes multiple instances of firefox are running on your computer and this may present conflicts go into Windows task manager if using XP:
Press Ctrl + Alt + Del simultaneously
Go to Processes
Click on Image Name at the top of thew column and it will sort the processes alphabetically
Select firefox.exe and hit End Process.
When presented with the Warning popup hit the Yes button.
Make sure you do this for all instances of firefox.exe that are running