I now have this working on my windows box. Assuming you are using
pymake this is what I did:
1. Delete your whole Firefox Source folder
2. Open MINGW32
$ cd Desktop
$ hg clone <your repo> FirefoxSource
$ cd FirefoxSource
$ autoconf-2.13
$ cd js/src
$ autoconf-2.13
$ cd ../..
$ mkdir ff-dbg
$ cd ff-dbg
$ ../configure
$ python -O "../build/pymake/make.py" -s -j10 #change j to about 2x
the number of cores that you use
Hope this helps.