They've been talking for years about written more and lower layers with Javascript. I'm not sure how far along they've got, it would be interesting to know.
Obviously parts are C++ but isn't interfacing with such code one of the defining characteristics of a "scripting language" and javascript in particular?
Vimperator and Conkeror do some pretty complicated manipulation in pure JavaScript. (Sure, the rendering core is in C++, but that's because C++ was all they had when it was first developed.)
i'm pretty sure that much of the performance-critical core is in C++; javascript is th UI/extensions layer, to the best of my knowledge