Mibbit is a web-based IRC client that lets you log onto pretty much any IRC server from the comfort of your web browser. Typically the first thing you need to do is visit mibbit.com. But that's about to change, because Mozilla is adding Mibbit support to the next version of Firefox.
If you download a recent nightly build of Firefox 3.5, you'll find a new option for "irc" in the applications setting in the options dialog. From the drop-down menu, you can select Mibbit or designate a desktop IRC client. Now when you enter an IRC server in your URL bar, Firefox should automatically launch Mibbit. Just make sure the address starts with "irc://" or "ircs://" and you should be good to go.