Adding components to XULRunner is simple once you understand how XULRunner registers and saves the components.
After a new component has been written it must be placed in the app/components directory (not app/xulrunner/components). Once it's in place all you have to do is run the stub to launch the application. XULRunner will automatically register any new components and log it in the current user's profile directory under compreg.dat and xpti.dat.
Where Vendor is the vendor line, and Name is the Name line in the application.ini file. xxxxxxxx is the random hash in your Profiles directory for the active profile.
Most of the problems you'll have creating a component, at least that I've had, is in the re-registering of components. Thanks to bsmedberg in the #xulrunner IRC channel, I figured out that all you have to do is either delete the compreg.dat and xpti.dat files in the profile directory or simply change the BuildID in your application.ini file.