This is not a solution because I don't have the problem, and have never seen it either, but it might be a workround you can live with.
Right click on the menu bar and untick it. It will be replaced with a small 'Firefox' menu at the start of the tab bar. See if you can work with that. A side effect is that it gives you ever so slightly more screen space for the main part of the window as well, so that is a bonus.
Fujitsu Lifebook AH532 Laptop. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Ubuntu12.10 (Unity), Mint14 (Cinnamon), Manjaro (Xfce).