I finally got around to playing with Greasemonkey user scripts and they are surprisingly easy to write. Basically, you just put the following into a JS file and install it:
// ==UserScript==
// @name Foo
// @namespace http://www.keebler.net/blog/
// @description Bar
// @include *
// ==/UserScript==(function () {
// YOUR CODE GOES HERE
})();
That’s only half the fun 
Next you can convert your script into a Firefox extension.
I took a great Greasemonkey script from here and generated this Firefox extension that displays a floating panel whenever you’re on a page with an RSS feed. Go forth and multiply, my extension children!