First off, let me tell you, it's a huge pain in da patookis. Making a very simple plugin isn't that big of a deal. Even scriptable from the original HTML page. Oh wait, but you want a user interface? Oh, now we'll have to haggle.
Here are some great resources for XPCom, the modular design that Firefox and it's bretheren are built on which allows plug-ins and add-ons to the browser without having to recompile the browser.