Strangely, in Firefox, this brings up a new tab with a Firefox help page. Another option for getting to the menu is the "Move focus to the menu bar" keyboard s

Strangely, in Firefox, this brings up a new tab with a Firefox help page. Another option for getting to the menu is the "Move focus to the menu bar" keyboard shortcut which defaults to CTRL-F2, but is assignable. Neither of these methods are as direct as the Windows way, as you have to use cursor keys to move to where you want, but it works.