Easy to reproduce on Linux, maybe less easy or not possible on NT (haven't tried). Start mozilla Click on a menu item in the menu bar (e.g., File). Without moving the mouse, click again (this should dismiss the menu), and then again, without moving the mouse, click again to bring up the menu. The menu will display, but the menu item in the menu bar (e.g., File) does not redraw to indicate the menu has been posted. Slight movement of the mouse will cause it to draw (because, I suppose, a redraw event is posted in response to the mouse movement). I will attach a gif showing how this looks
This happens on WinNT just like on Linux, one (possibly) usefull detail: Waiting for more than about a second after dismissing the menu before bringing it up again does display the menu item, so this might be a timing/event issue.
Added some cc's. Timing, hmmmm.
*** This bug has been marked as a duplicate of 13334 ***