Similarly I would expect that generating any sort of sensible Javascript would require something fairly tightly tied to GHC; otherwise the output's going to have horrible performance because it's not going to understand the input and will fall back to the slowest but most general translation. (If it even has such a fallback, instead of simply failing on code that it doesn't recognize.)
Emscripten produces a limited subset of JavaScript (asm.js), and Firefox has an optimizer specifically for that. It could result in good performance, but the CPS transformed code is still a problem indeed, so it's unclear if GHC generated code would work at all. Asm.js is really limited, it's like a far less convenient LLVM without modern instructions in JavaScript syntax. You can't use objects, everything is stored in a global heap array. Interestingly, Firefox performs far worse than Chrome and Safari on the current GHCJS code, probably because their garbage collector is rather poor (non-generational), and Haskell code produces lots of short-lived immutable objects. luite