If it's the toolbar that has the "File", "Edit", "View", "Go", Bookmarks", "Tab", etc... That one is hardwired as far as I know.
[edit]
O.K. I just figured out why, it's because ya moved all yer icons up to the top bar. That bar that ya left empty is the leftovers of that bar and ya can't get rid of it. It seems that you can not move the entire bar to the top one, only an icon at a time. I just tried it, sorry. 🙁
Probably the only way would be to go into the coding or a CSS, maybe the userchrome.css Or something to do with the overlay. Not sure, me still at work sozz me just throwin things out there. 😀