ChatZilla is an IRC client, written in XUL and JavaScript for Mozilla web browsers.

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 features of IRC clients, such as connections to multiple servers, IPv6, SSL and UTF-8. It supports JavaScript as a 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. Features such as DCC, which allows users to transfer files between one another, are now supported.

Work on ChatZilla progresses, and ChatZilla is currently included with the Mozilla Application Suite and 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.