Creating New Packages for Mozilla
The final step in the tutorial is to create a XPI archive in which the install.js script and the barley.jar can be redistributed. The archiving of an archive may seem a little redundant–and if you want you can instead use the XPI to archive the install script and “flat”, or uncompressed, versions of the resources.

Good artilce on how to create a XPI for Firefox or Mozilla. Handy if you are into customizing your Firefox extensions. ;)

How to write Firefox extensions using BugMeNot as Example.
Everyone has a good idea at one time or another to implement a new feature in a web browser. Well, with the goodness that is Mozilla Firefox, now you can do just that. You need to have a vague understanding of XUL and Javascript, but you certainly don’t need to be a master of either. When I started, I knew nothing about either one, really. I had seen some bookmarklets here and there, and tried to figure out just how they worked. Well, that’s how I made my first extension, BugMeNot.

Here is a good article with some helpful info on getting started with writing your own Firefox extension.