First download the source of course. I follow the mozilla build instructions for this.  
First create a file called .mozconfig in the source see below edit to your needs.  


#### .mozconfig

# sh
# Build configuration script
#
# See http://www.mozilla.org/build/unix.html for build instructions.
#
# Mozilla FireFox build options:

export MOZ_PHOENIX=1
mk_add_options MOZ_PHOENIX=1

ac_add_options --enable-optimize
ac_add_options --disable-debug
ac_add_options --prefix=/usr
ac_add_options --with-default-mozilla-five-home=/usr/lib/mozilla-firefox
ac_add_options --enable-strip-libs
ac_add_options --disable-tests
ac_add_options --disable-short-wchar
ac_add_options --enable-nspr-autoconf
ac_add_options --enable-extensions=cookie,xml-rpc,xmlextras,pref,transformiix,universalchardet,typeaheadfind,webservices,inspector,gnomevfs,negotiateauth
ac_add_options --enable-crypto
ac_add_options --disable-xprint
ac_add_options --without-system-nspr
ac_add_options --with-system-zlib
ac_add_options --enable-default-toolkit=gtk2
ac_add_options --disable-freetype2
ac_add_options --enable-xft
ac_add_options --enable-xinerama
ac_add_options --enable-xterm-updates
ac_add_options --disable-accessibility
ac_add_options --disable-activex
ac_add_options --disable-activex-scripting
ac_add_options --disable-composer
ac_add_options --disable-installer
ac_add_options --disable-ldap
ac_add_options --disable-mailnews
ac_add_options --disable-profilesharing
ac_add_options --enable-single-profile
ac_add_options --enable-static
ac_add_options --disable-shared
ac_add_options --enable-official-branding


Now we issue these commands.
gmake -f client.mk checkout
gmake -f client.mk distclean
gmake -f client.mk build

Now you can either issue a make install or a make install DESTDIR=/tmp/package-firefox

To create the package some of the post install steps are.
Create the desktop files and put the pixmaps in the correct location.
Desktop files in /tmp/package-firefox/usr/share/applications
pixmaps in /tmp/package-firefox/usr/share/pixmaps
Create a slack-desc file for it and put it in /tmp/package-firefox/install

Additionally 
I remove the pkgconfig files and that folder not needed 
nothing is going to look for firefox anyway.

I also remove the /usr/include folder since we built static 
no need for these and again nothing is going to use these anyway.

I also rename the /tmp/package-firefox/usr/bin files to mozilla-firefox just 
to keep it standard with the other packages I have built.

And to reduce the size I remove the aclocal and idl directories that are 
in /tmp/package-firefox/usr/share

I would guess that if firefox ever becomes the default browser you can 
keep all those things I have been removing but for now its a bunch of cruft.