Yesterday I had a single Firefox window (with approx. 20 tabs) open, and then the window disappeared. I figured that I'd lost all my tabs. When I opened a new window, and attempted to check my Outlook mail, I got the message "There was a problem opening your mailbox. You may have already signed in to Outlook Web App on a different browser tab. If so, close this tab and return to the other tab." I am now hearing a *ding* whenever I get a new email. However, I cannot find the window or tab in which my Outlook is open! It's like the Firefox window is invisible or hiding somewhere. If possible, I'd like to locate this invisible Firefox window, and not have to re-locate all the websites I had open in the tabs. I've opened the Windows Task Manager, but it doesn't show any open Firefox applications. Any ideas? Where is this open Firefox window hiding?