Have a window with many-many tabs, say hundred(s). Have the "Show all tabs" configured to show tabs as grid of previews. Open Task Manager and display "GDI Objects" column, note the number for Firefox. Now do "Show all tabs" (e.g. via Ctrl+Shift+Tab) for the first time after Firefox restart - you will see that GDI Objects count jumps dramatically. It can easily hit the OS limit of 10000 GDI Objects per process (Firefox windows will freeze then). Even when you close the "all tabs" panel, the GDI Objects count doesn't drop to the previous low level. I don't know how many GDI objects each preview takes, but it's way too many and they are not returned to the OS.
confirmed here: https://blog.mozilla.org/nnethercote/2012/07/19/firefox-15-plugs-the-add-on-leaks/comment-page-1/#comment-6886 Just tested again on FF 17.0.1 - the behavior didn't change.