Hi, have you tried clicking on view, toolbars, and uncheck the menuitem box. Or simply remove toolbars one by one to see which is causing the problem. I'm not a Mozilla expert just a suggestion, hope it helps.
You have a conflict with an add on, or a theme. Try running firefox without all the add ons or themes, and see if the problem persists.
You can either disable your add ons or themes one by one until you find the problem, or uninstall firefox, re-install, and run plain.
You can also launch firefox in safe mode from start > programs > Mozilla Firefox, and see if the menu item statement shows up.
