Hi Tim,
As Rolando mentioned, you can build SpiderMonkey with dev tools from
xcode (clang instead of gcc, etcl), but you need autoconf 2.13, which is
usually installed via brew.
If you're for pre-built binaries, you can pull one down from TinderBox
directly:
Visit
https://tbpl.mozilla.org/
Click on a green "B" immediately to the right of one of the "OSX 10.N
opt" builds, depending on what version of OSX you're running.
At the bottom there'll pop up a pane, and on the left hand side of the
pane there'll be a link called "go to build directory"
Follow that link, and it'll take you to a directory where you can
download firefox and spidermonkey shell builds associated with that
revision.
Please remember that these are nightly builds, and contains changes that
have not gone through the aurora/beta release train, and there may be
undiscovered bugs in the build.
Cheers.
Kannan