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.
about:config where you need a search engine to find all the key strings does not count as easy in this context. And it's unreasonable to pretend it is.
I'd argue that you shouldn't need third party add-ons plus modifications to both userChrome and about:config to do it, so it could be easier. A "Customize Context Menu" under Edit would be nice and easy for even regular users to discover and take advantage of.