So you want to create a Mozilla Firefox extension huh? Well about a week ago, so did I. This chronicles my adventures in learning how to do so.
The first thing you need to do is Create the obligitory Hello World extension, and get it working. Read over what they have written in the Mozillia Wiki, if you need some help, or if something confuses you, you might find what I wrote here helpfull. I would recommend having them both open while you do it. Just take it step by step. What they say contains alot of theory (at the time I read it it did, although it can change daily), while what I wrote is just basic step by step directions to get you started. The most imcomplete part on their page is the actual install. When I read it, they said about 2 sentances on how to do it, and never really explained it. I give step-by-step directions.
Finished the Hello World extension? Good! Now you might be interested in reading some more theory and such, so go ahead and browse through the Mozilla Developer site, and get a little more familiar with it. When you find something useful, just make a bookmark of it so you can find it later. Here are a few sites I have found useful myself:
Extend Firefox
tutorial
XUL Element Reference
Those two links right there (especially the last one) will help you solve alot of your problems.
If you still need a little more insight into the process, you can see step-by-step (with screenshots, and the full code for each step) EXACTLY how I created my first extension. The Calculator Extension. It's not finished yet, but it is 80% done, and definately has enough information to get you either thoroughly confused, or well on your way to becoming an extendor yourself.
Hopefully there will be more extensions to come in the future, as well as some themes.
Have fun, and e-mail me with questions. cmcculloh at google mail.