http://www.instructables.com/id/Extract-Firefox-Addon-Source-Code/

1. Locating an Addon's XPI File. An XPI file is an addon packed into a single file. https://addons.mozilla.org/ -- "Save Link As..."
2. Extracting XPI File
   
   install.js
install.rdf
/chrome/
/defaults/
3. Extracting the Main JAR Code. Most of the core addon code is present in a JAR file found in the /chrome/ directory.

Opening XPI files in Firefox will install them for you.