Why? There's no real advantage to having an IRC app in a browser. Mozilla has it as a third-party add-on, which is as it should be, but it hasn't really got any advantages because of being added into a browser. You could as easily build a standalone app in the Gecko Runtime Environment (Snowbird, anyone?) and there wouldn't be a single disadvantage over the browser-extension version.
If you want an IRC app that uses WebCore, though, Colloquy might fit your needs.