We're still defaulting to 32-bit on systems where both are supported, but we hope to default to 64-bit around Firefox 55. We had to delay this due to issues with Flash, sandboxing, and Farmville 2: https://bugzil.la/1317995
Yuhong, is there a bug filed for the x64 SpiderMonkey JIT issues? What is the side effect of not following the x86-64 ABI for JIT code?
SEH exception handling don't work properly and debuggers don't show a proper callstack. For it to work properly, the JIT has to generate "function tables" to allow Windows to do stack walking.