John Gruber accuses Firefox of being a "Bad Mac App"...
It looks like you can customize menu key shortcuts using the Keyboard Shortcuts panel in System Prefs, but the custom bindings don't work, and the factory shortcuts continue to apply, even though they no longer appear in the menu bar. He's obviously never tried to do the same thing in Safari, where though the custom binding do work, the factory shortcuts also continue to apply, even though they no longer appear in the menu bar.