How to change Firefox's context menus (and other menus)

I have long wanted a way to disable the "Send Link as Mail" item on Firefox's context menu. Never have I meant to use this feature, and whenever I do accidentally click on it, the computer freezes while some unwanted mail client is launched (I never bothered to configure the mail client either).

Thankfully, here is a very simple solution. Use the Menu Editor add-on:
https://addons.mozilla.org/en-US/firefox/addon/710

Install it, restart Firefox, then use Ctrl+Shift+S to launch the Menu Editor.

Before the advent of this tool, you had to mess with css/chrome settings which looks like a pain.