I have a Macbook Pro running 10.6.8 and Firefox 5.0.1 I have had this issue for a while and was hoping that the newest release would solve my problem, it didn't. I looked online for solutions, none of which fixed the issue *Some pointed the finger at the extension Invisible Hand, which I never had. *Some suggested putting Firefox into safe mode, this solves the problem, but since it stops me from installing any extensions this is not a workable solution. *It was advised that I create a new profile, which I tried, this did absolutely nothing, the blank page appeared again after a short period of time and still can't be closed or clicked on, and this was before I added any add-ons. Which leads me to believe that the issue lies with Firefox. Though on a side note when I quit Firefox, with only my current tab and the "tab that can't be closed" still open, Firefox notices that there are two tabs open and warns me that I am going to be closing both. The only work around I have found is to set the "tab that can't be closed" as an app tab for the current session, but when I open Firefox the next time, the app tab which was the "tab that can't be closed" can now be closed and a new "tab that can't be closed" appears as if it were a new blank tab, just like every other time I open Firefox.