Classilla is a Gecko-based web browser for PowerPC-based classic Macintosh systems, essentially an updated descendant of the now-defunct Mozilla Application Suite by way of the Mac OS port maintained in the now-aborted WaMCom project. The name is a portmanteau of Classic (the classic Mac OS, as defined by the Classic Environment), and Mozilla.

Like the Suite it is descended from, Classilla offers E-mail (POP/SMTP), Usenet (NNTP), Gopher, FTP and World Wide Web (HTTP) access, using a modified version of the Gecko layout engine called Clecko. Classilla also includes its own versions of the DOM Inspector, Mozilla Composer and Venkman components; the former IRC ChatZilla component was removed in version 9.1. Classilla is currently the most recently updated major browser for classic Mac OS systems, and the only Mozilla-based browser for that environment in current maintenance as well, as iCab 3's final update was 3.0.5 in January 2008,Opera's Mac OS 9 support ended with version 6.03 on 20 August 2003,Internet Explorer for Mac on the classic Mac OS ceased development with 5.1.7 in July 2003 and Mozilla itself ceased support in 2002 (see History). The primary maintainer is Cameron Kaiser.