Eric at Roachfiend.com has created an excellent beginners guide to creating your own Firefox Extensions (now called add-ons for you beta users). I won’t go into all the details here, since he already did a great job explaining it.
If you have ever wanted to build your own extension, it is not as hard as it may seem. I may have to try it out for myself soon. I have a few ideas I would like to implement in Firefox, and this would be the way to do it.