Comment 10 for bug 161961

Citing from http://www.shortcutworld.com/shortcuts.html?id=firefox

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Accessing and working with the Firefox Menu Bar:

alt or f10: Jumps to the menu bar. From here you can use arrow left, arrow right, arrow up and arrow down plus enter to navigate and activate between the menu items on top of the browser window.

This works at least in gutsy, but as stated: f10 only activates the menu-bar, to open a given menu you'll need to use the arrow-keys. Don't forget that firefox is not a native gtk/gnome application, so there will be always a few differences.