LifeHacker has a great how-to article on building Firefox extensions. They make it sound so easy I might just have to give it a try.
Speaking of Firefox extensions, here are a few of my favorites in roughly the order of usefulness they have to me:
NoScript - allows you to choose which web sites are allowed to run scripts in your browser. Since scripts have huge potential for delivering malicious software to your computer, this is very important indeed.
Google Browser Sync - This provides an easy way to keep your browser bookmarks and settings synched up between multiple machines.
Book Burro - This plug-in senses when you're looking at a book on one of several web sites to purchase books. Once it realizes you're looking for a specific book, it presents you with the prices at several web sites so you can find the lowest price. The list of sites it checks is configurable.
Long Titles - This prevents long tooltips windows from being truncated. Since a number of the web comics I read make use of tooltips to add hidden messages, this is very handy to have.