Hi,
The way that I view plugins is to add large scale functionality to the
browser that does not currently exist (e.g. Java support, Macromedia flash,
etc… NOTE: these are examples only these products currently work with Fire
Fox). The extension model is more useful for enabling smaller functionality
to the browser (e.g. mouse gestures).
Since this is a pet project I would suggest not concerning yourself with
writing a plug-in it would be much more productive to write an extension.
Since extensions are written using XUL and the implementation is
comparatively easy you can probably pop one out just by walking through a
tutorial or two. You should be able to add most any functionality that you
desire with this process. Just take a look at the extensions Adblock, Web
Developer .8 , ieview, and Mouse Gestures 1.0
(
http://optimoz.mozdev.org/gestures/index.html) and you will see that a lot
is possible with the extension model that Mozilla has provided.
Good Luck!