The source code for the After the Deadline Firefox add-on is hosted on a public SVN. You can checkout the source with:
svn checkout http://openatd.svn.wordpress.org/atd-firefox
There are opportunities for you to get involved with the development of this add-on or After the Deadline. Read the After the Deadline Get Involved Guide to learn how to get an account on TRAC (our bug system), join our mailing list, and other rules of engagement.
Specific to this project: