ChatZilla is a clean, easy to use and highly extensible Internet Relay Chat (IRC) client, built on the Mozilla platform, which provides all the usual features: multiple servers, a built-in list of standard networks, easy searching and sorting of available channels, logging, and DCC chat and file transfers, plus easy customization with JavaScript plug-ins and CSS styling.
ChatZilla is also available in Catalan, French, German, Italian, Polish, Portuguese, Russian and Spanish.

Go to this page and click to install this add-on.
Restart firefox,launch this from Tools->ChatZilla.You’ll get this help about how to use this chat client.