In Firefox, on Linux at least, when you press Alt you get a bar with all the standard File/Edit/View/History/Bookmarks/Tools/Help menus. In each menu and then in each item there's one letter underlined and it works as it should, so a sequence of Alt, h, a, will give you an About dialog (for example). The fact that the menu bar is not displayed by default doesn't bother me, because I don't use mouse to navigate the menus, and from keyboard I'd need the Alt key press anyway.

Slightly related: somehow, FF on Linux allows you to rearrange tabs from keyboard. Ctrl+Shift+PgUp/PgDown move the current tab left or right. As far as I can tell, this feature is only available on Linux. I have no idea why. It's much more convenient than dragging the tabs with mouse, because you don't have to worry about accidentally detaching the tab into its own window.