Nope, that isn't normal.

Do you have that problem when running in the Firefox SafeMode?
A troubleshooting mode.
You can open the Firefox 4.0 SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut. 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. I have restarted FireFox in safe mode and checked that all add-ons are disabled but I still have the problem. I read that some disabled the Yahoo toolbar and that solved their problem. I am running the Google toolbar and have uninstalled it but that did not solve my problem. I simply have to click twice on the back button when multiple tabs are open.