Did you ever have a great idea for a Firefox extension but didn’t know how to do it, well it isn’t that hard! To begin you only need some knowledge of javascript, which is even required since it isn’t that hard ;) There are 2 options, a “simple” way and a “hard” way, of course both have their advantages and disadvantages.
The simple option uses the greasemonkey extension. The advantages of greasemonkey is that you don’t have to deal with installing an extension and restarting Firefox over and over again. Also you only need to care about your script. Greasemonkey allows you to install javascript files which you can apply to all sites, or only certain site. You can check out how to make scripts for greasemonkey or browse an collection of greasemonkey scripts.
The hard option is making an extension directly. This involves making scripts like with greasemonkey, but also adding files which tell Firefox the details about the script. Downsides are that you have to repack and restart every time you modified your script (i believe there was a solution to this, but i don’t know for sure)
Upsides are that you can do a lot more then you can with greasemonkey and you can distribute your extension a bit easier.
You can find a nice tutorial about creating extension at roachfriend.com.
Also check out the links at the bottom of the tutorial if you get stuck, the forum he links to for example can be a big help.
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You can use http://www.arantius.com/misc/greasemonkey/script-compiler.php to compile a GreaseMonkey script into a full-featured independent Firefox extension!