Add an Arbitrary Custom Header to Email in Mozilla or Netscape

We usually think recognizing people is remembering their faces, but have you noticed how you can spot somebody in a group of people by the way they walk, often long before you get to see the face?
By default there's nothing distinctive about the heads of your emails (except for your email address maybe...), but you can make them recognizable — albeit not from very far away — using custom headers:
Want to add your fancy "X-Y: Z" header to emails or need to set the "Sender:"? Here's how to add any header to emails you send with Mozilla or Netscape.