Firefox 3.5 will support IRC through Mibbit
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.