Hi,
Whenever I accidentally press the Alt key, I am surprised that FireFox still retains the menu bar. No other major browser has the menu bar any longer and I, for one, don't ever use it. I guess many others don't, either. I believe you could make FireFox leaner, more resource efficient, and improve it's performance, if you removed the menu bar altogether. Anyway, I don't see any purpose to keeping it, especially since the side menu practically contains all of its options and actions. JMTC.
Good Luck.