Looks like SquirrelFish is already “dead”. There’s now SquirrelFish Extreme (SFX). You can see some benchmark comparisons here.
In the past few months, TraceMonkey, V8 and SquirrelFish.
I suspect the next place is going to be improving DOM performance. At some point JS performance (in feel) is going to be limited by the ability to display what’s been processed. Part of that will be DOM speed. There’s also gfx speed, which Mozilla has already done some good work in prepping for. Utilizing Cairo has been a good start. There’s discussion of OpenGL and taking advantage of the GPU. Once this “my JS is faster than yours” phase is over, that’s where I expect things to go.