Plugins for other webmail clients.

Hey guys,  I'm wondering how hard it would be to develop new "plugins" essentially to other webmail clients such as yahoo or whatever else I might want to use.  I'm interested in writing one for my web-based email and was wondering what would be involved.  I have downloaded and begun to look at the source code, and notice the gmail.js and gmail2.js scripts,  I am wondering if it could be as easy as adding my own script for my webmail client there, and then I guess figuring out how to add a tab for that in the preferences window.  I'm a fairly experienced developer, but have no real experience with firefox plugins.  Any help would be appreciated, thanks!