The Add-ons Manager is probably one of the most used menu entries which is not accessible from the Firefox button. And as long as bug 465090 is not fixed we also don't have a keyboard shortcut. That will force users to always disable the button in favor of the default menu bar.
Its the customize->Addons on Windows.
It is there in Customization submenu.
My fault. Sorry. Simply haven't it expected at that position.