It literally already is fully customizable. between userChrome, about:config, and extensions, you can do literally anything you like to your right click menu on Firefox.
Just kidding, but it does illustrate that there's always a tradeoff with these things. (I would like to have the ability to customize the context menu too, fwiw, though it's not as straightforward as the other customizable bits of UI since the context menu is, well, contextual.)