From the Release Notes:
interpreter it uses is almost exactly the same as Mozilla Firefox 3.0.19.
This literally cuts its benchmark time in half (the 1.8GHz dual G4 build host
reduced SunSpider from 32 seconds with 9.1 to 16 seconds with 9.2; the quad
G5 in Classic went from 25 seconds to 12 seconds [for comparison, Firefox
3.6.3 on the same machine with TraceMonkey is 4.6 seconds]). It also repairs
more stable on sites like Blogspot and iFixit."