I ran into this while working on a patch for bug 569342, but I've discovered it's a more general problem. You can reproduce this by loading an image in the browser. The find command keyboard shortcut and menubar menuitem will be disabled, but the menuitem in the appmenu is not disabled. I suspect this may be a problem with the fact that the menuitem in the appmenu is the child of a vbox instead of a menupopup, so some xul widget code may be getting messed up somewhere, but I'm not sure.