Am running FF4b9 on XP. Occasionally when I have multiple tabs, I can press Ctrl-Shift-T to undo a closure of a tab recently closed. When I do this however, it hides all open tabs and presents me with a new window with JUST the recently recovered tab in it. If I close that, the previous window with all the other tabs reappears. If I use panorama to go to a previous tab, the new window with the unclosed tab vanishes, not viewable again. I can't give exact steps to make it happen, but when it starts happening, it consistently occurs after that.
Will be fixed in beta 10.