js/src directory. This can't be right, but I give up reading the Mercurial doco.
Once you get the source code, it needs to be autoconf'ed with
autoconf-2.13 and not whatever the latest autoconf on your system is. I don't know why.
Then there are lot of directories you don't want (test files, misc integration tools, etc). Stripping those out takes some time too
I've done all this mess, the results is here: libmozjs-2009-01-12.tar.bz2. Enjoy!
My latest packaging script is: make-js-tar.sh