it's the step "above" that IE. everything is in binary but don't have the actual
architecture instructions which means the browser have to do one compilation
before it can run it.
Google recently released a benchmark suite called Octane 2.0 which includes
one Asm.JS test (the zlib one). On my computer the results on that test are:
Firefox (which have Asm.JS support): 35788
Chrome (Without Asm.JS support): 12147
So pretty good optimisations are made. Overall however Chrome beats Firefox.