What happens when you click on the close button in Firefox 4?

Can you try moving the browser window, to see if there is an alert window or something that is hidden behind it?

Hi, thanks for asking. Actually, nothing happens when I click on the close button. I can just keep clicking away and the browser stays open. The same thing happens if I try to close my browser from the task bar. There is no message behind the browser window. A very odd thing but it makes Firefox 4.0 nearly unuseable for me since I have to either reboot my computer or shut it down to close the browser.

Does this still happen if you restart Firefox in "safe mode"? For instructions, see: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.