Here are the steps I used:
1. get client.ml
$ cvs -d :pserver:anonymous@cvs-mirror.mozilla.org:/cvsroot co mozilla/client.mk
2. sync firefox code
$ cd mozilla
$ make -f client.mk checkout MOZ_CO_PROJECT=browser
3. create .mozconfig
. $topsrcdir/browser/config/mozconfig
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/ff-opt-static
ac_add_options --enable-optimize
ac_add_options --disable-debug
#ac_add_options --enable-static
#ac_add_options --disable-shared
#ac_add_options --disable-libxul
ac_add_options --disable-tests
4. install build dependency libs
sudo apt-get build-dep firefox
5. install curl
sudo apt-get install libcurl3-dev
6. install libdbus-glib-1-dev
sudo apt-get install libdbus-glib-1-dev
5 & 6 must be done, or else the build script will report error on missing curl.h or dbus-glib is too old
7. kick off build
make -f client.mk build
