ChatZilla is a "clean, easy to use and highly extensible IRC client, built on the Mozilla platform". It is written entirely in JavaScript, supported by XUL, CSS and HTML.

It is designed to run on any platform on which Mozilla runs, such as Mac OS, Linux, BSD, Microsoft Windows, Solaris, IRIX, BeOS, AIX, HP-UX, and OS/2. It currently supports most standard features of IRC clients, including connections to multiple servers, a built-in list of standard networks, easy searching and sorting of available channels, logging, DCC chat and file transfers, plus easy customization with scripts and styles. It also supports IPv6[citation needed], SSL, and UTF-8 by default. JavaScript is used as the scripting language. Messages are styled using CSS, which means that the style of the client can be easily changed, including one that places photographs next to the names of users who speak in the channel. DCC, which allows users to transfer files and chat directly between one another, is supported but with no resume for file transfers.

ChatZilla is currently included with SeaMonkey as well as being available for Mozilla Firefox as an optional extension. It is also available as a standalone version which runs on top of XULRunner.