Well, on System 7, XULRunner has about as much chance of getting off the ground as WebKit (little to none). I think Theos was talking more about options for earlier versions of OS X -- you can use TenFourFox's patches to build a 10.4-compatible XULRunner, for example. Classilla works on OS 8.6-9 because it's a hacked Mozilla 1.3.1 which already worked on OS 9 to begin with. You might be writing your own app-specific renderer for this use case.

@commodorejohn, there are some very good CodeWarrior/PowerPlant books for the classic Mac OS. Dan von Sydow wrote one, but the title escapes me.