I suggest the following:

• Go to Download Firefox 4.0 and download a fresh installation file to the desktop.

• Then go to Add/Remove Programs, scroll down to Mozilla Firefox and remove it, choosing to keep your bookmarks, customizations etc., (don't checkmark the box).

• Then reboot and delete the folder called Mozilla Firefox at this location: C:\Program Files\Mozilla Firefox

• Finally run the installation file you downloaded.

Thanks for the suggestions - unfortunately I can't download Firefox 4.0 as it requires that Firefox be closed first (despite the fact that it's not running). The same problem exists with the uninstall. Any other ideas would be appreciated.

You can download it with IE8 or 9. Just choose to save the file to your desktop instead of clicking "Run".

I have now downloaded Firefox 4.0. However, I still can't uninstall Firefox 3.6 as I get a message that I must first close the version that is running. I have tried working with task manager but it doesn't show any firefox version running.

Press CTRL+SHIFT+ESC to bring up Windows Task Manager, go to the Processes tab and you should be able to find firefox.exe in the list if it's still running. Then right click it and choose "Kill Process".

Success. I'm now up and working with Firefox 4. Many thanks for your help.

You're welcome.

Could you please click the "Solved" button so that your post gets taken out of the loop?

Thanks.