A Google for "Firefox 4 tab problem" suggested that the problem was caused by "Ask Toolbar" being incompatible with Firefox 4! I was not even aware I had the "Ask Toolbar" loaded! A forum thread offered the following solution:-
These are the steps to uninstall the Ask Toolbar from Windows XP, Windows Vista and Windows 7:-
For Windows XP:
- Make sure all the Web browsers or there processes are closed.
- Go to Start menu and Select “Control panel” and then select “Add/remove Programs”
- Search the Ask Toolbar from the list of programs installed (or the program with Ask logo)
- Click on Change/Remove to uninstall it.
- Always close all the instance of Web Browsers or related programs.
- Go to the following path Start > Control Panel.
- Under Programs icon select “uninstall a Program”
- Now Select the “Ask Toolbar” from the list and click on “Uninstall”
- Go to the following path: Start > Control Panel > Programs > Uninstall a Program.
- Select the Ask Toolbar from the list of installed programs and right click to Uninstall.