When you have opened so many tabbed panes in a single window that each tab has reached its minimum size (just big enough to show the loading/bookmark/page icon) and you then continue to open tabs, these further tabs are opened off to the right of the window bounds without visual indication or any way to get at them without deleting existing tabs. Contrast this with filling the personal toolbar folder with too many bookmarks, where you get a >> double chevron symbol indicating that there is more to the right, and can click the double chevron symbol to get a pop-up menu of what you can't see. The double chevron list updates when you resize the window, too. Tabs needs its own double chevron. As an aside, opening all new tabs at the extreme right of the tabbed pane list is silly. Each tab should be opened to the immediate right of its parent tab, so that you can stack and group them automatically.

*** This bug has been marked as a duplicate of 155325 ***