Mibbit Browser is a very easy to use Firefox add-on that opens up a separate window that gives you the possibility to use the Mibbit IRC client while browsing.
Mibbit Browser add-on creates a Mibbit! entry in the Tools menu that opens up a basic window to Mibbit that allows you to connect to the Mibbit IRC client while browsing.
Mibbit Browser will integrate quickly into your Firefox and you will be able to easily manage it via the Add-ons window that you can access through the Tools menu.
To learn more about dealing with Firefox add-ons you can access the Mozilla page HERE.
Note: Mibbit Browser add-on is cross-platform and it works on Mac OS X, Windows and Linux.