I don't think the actual deskbar sources will help you. The answer lies in the Firefox sources (which scares the **** out of me :-D).
If you want to look at the deskbar sources here's how: You can do an svn checkout as decribed here:
http://live.gnome.org/DeskbarApplet, and you will find the sources in deskbar-applet/deskbar/handlers/mozilla.py. Otherwise you can just look directly in the installed files: /usr/lib/deskbar-applet/handlers/mozilla.py. The class Firefox2SearchEngineParser is the one doing the parsing of the firefox search engines.
On the actual plugins. The user-installed ones are located in ~/.mozilla/firefox/<your-profile>/searchplugins/ and the global system ones are in /usr/share/firefox/searchplugins.