For more information, I decided to go to the source and write to Cameron Kaiser, the mind behind the project. He graciously replied and provided some clarifications:
Mac WaMCom was based on Mozilla 1.3.1, after which OS 9 support ended. Because Classilla no longer corresponds to a particular Mozilla release despite being based almost completely on Mozilla code, it uses its own layout track named Clecko (instead of Gecko). If you forced me to say, I would call Classilla 9.0 roughly equivalent to Mozilla 1.6 (Firefox 1.0), but there are some bugs in Classilla that Firefox 1 does not have, and conversely some patches from as late as Moz 1.8 (FF 1.5) are also landed, so it is really a distinct engine despite its origins.
This version is deliberately unfinished because there is no source for WaMCom anymore, so I assigned an arbitrary deadline of July 1st to myself to get an update out. It should be considered in that light. I think people will find the improvements noticeable, but no miracles should be expected from Classilla for awhile, and it still has a number of bugs.
The version numbers, by the way, are based on OS 9 releases. So the next one will be 9.0.4, and then 9.1, etc.
I’m looking forward to try Classilla on my original blueberry iBook G3 clamshell (with Mac OS 9.2.2) and PowerMac 9500 (with Mac OS 9.1). If I’m not completely drowned by work, expect a brief review in the following days. I’m really glad something is still moving in pre-Mac OS X land.