Probably caused by an incompatible extension.

Do you have that problem when running in the Firefox SafeMode?
A troubleshooting mode.
1.You can open the Firefox 4.0 SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
2. Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut to open it again.

If not, see this:
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Thanks for your reply. It doesn't appear when I open in safe mode, but it does with your second option.

All seems OK now apart from losing my addons. Think it was the theme which caused the problem. There doesn't appear to be an update for it.

Many thanks