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 IRCChatZilla 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.
In May 2009, Cameron Kaiser announced his intentions to start porting later Mozilla updates back to the 1.3.1-based version used in WaMCom, christening his modified version as Classilla. This first version, given the version number 9.0 to match Mac OS 9 (with subsequent numbers matching OS 9 version numbers), was released on 30 June 2009. Version 9.0.4 was released on 28 October 2009. Version 9.1 was released on 26 February 2010. Version 9.2 was released on 4 June 2010. Version 9.2.1 was released on August 22, 2010. Version 9.2.2 was released on March 29, 2011.
Because of the aging nature of classic Mac OS systems and the availability of mobile web sites with functionality similar to desktop Web versions, version 9.3.0 changed the browser to report a mobile user agent by default upon its release on January 20, 2012.
Apart from its upgraded support for Web pages, Classilla supports most of the same features that Mozilla of the same generation did, with similar feature sets and bugs in its support for E-mail, Usenet, FTP and Gopher, although the latter received token upgrades. In a likewise fashion, Classilla also inherits many of the security failings of earlier versions of the Application Suite, many of which are still not patched and openly warned of by the developers. The presence of NoScript, along with the unusual nature of the classic Mac OS, is thought to add some level of protection, although it is the avowed goal of the developers to reach security parity with current Mozilla-based releases and repair outstanding bugs.
Classilla 9.3.0 introduced the Byblos HTML rewriting engine that can mangle individual pages at the source code level according to user-provided "stele" scripts, with the intent of lightweight adaptation of complicated content to the capabilities or quirks of the browser.