Composer is dead. Long live KompoZer, *crash* KompoZer has crashed.
Just a small note, Composer on 1.8 is horribly busted on all OS's.
Compiling KompoZer, http://www.kompozer.net/ went easy enough...
It loads but it crashes as soon as something is typed.
It also crashes on exit.

Note: it appears the project is dead or very idle

In mozilla/composer/app/Makefile.in, this was added:
( i didn't bother checking if it actually needed -lgame though )
ifeq ($(OS_ARCH),BeOS)
BEOS_PROGRAM_RESOURCE = $(srcdir)/apprunner-beos.rsrc
ifdef BUILD_STATIC_LIBS
OS_LIBS += -ltracker -lgame
endif
endif

For the mozconfig, i edited mozconfig.linux , ripped out some parts and dropped in a block of common options for BeOS:

# sh
# Build configuration script for LINUX
#
# See http://www.mozilla.org/build/unix.html for build instructions.
#
export MOZILLA_OFFICIAL=1
export BUILD_OFFICIAL=1

export MOZ_STANDALONE_COMPOSER=1
mk_add_options MOZ_STANDALONE_COMPOSER=1


# Uncomment the two following lines if you want the Site Manager
# to use KDE mimetypes icons. Warning, experimental feature...
# You may have to edit the following files
# mozilla/composer/app/Makefile.in
# mozilla/editor/libeditor/html/Makefile.in
# mozilla/composer/src/ *
#export SITE_MANAGER_KDE_ICON_STYLE=1
#mk_add_options SITE_MANAGER_KDE_ICON_STYLE=1

# Uncomment and edit this line if you want to build
# outside of the source tree
mk_add_options MOZ_OBJDIR=/preR1/KompoZer-bone

# Comment out the following lines for a debug build
#ac_add_options --enable-optimize
ac_add_options --disable-debug

# You may need to uncomment and edit that line. On Linspire,
# just uncomment it
#ac_add_options --with-default-mozilla-five-home=/usr/lib/mozilla

# You probably have nothing to change below that line
ac_add_options --disable-svg
ac_add_options --without-system-mng
ac_add_options --without-system-png
ac_add_options --disable-ldap
ac_add_options --disable-mailnews
ac_add_options --disable-installer
ac_add_options --disable-activex
ac_add_options --disable-activex-scripting
ac_add_options --disable-tests
ac_add_options --disable-oji
ac_add_options --disable-necko-disk-cache
ac_add_options --enable-single-profile
ac_add_options --disable-profilesharing
ac_add_options --enable-extensions=wallet,xml-rpc,xmlextras,pref,universalchardet,spellcheck
ac_add_options --enable-necko-protocols=http,ftp,file,jar,viewsource,res,data
ac_add_options --disable-pedantic
ac_add_options --disable-short-wchar
# // I don't want to do strip yet
#ac_add_options --enable-strip-libs
ac_add_options --enable-crypto
ac_add_options --disable-mathml
ac_add_options --with-system-zlib
ac_add_options --enable-toolkit=beos
ac_add_options --enable-default-toolkit=beos
ac_add_options --disable-freetype2

# Note: Only include icon for mac, windows and os2
ac_add_options --enable-image-decoders=default,-xbm


### // COMMON BeOS mozconfig stuff not found above.

### //EDIT THESE VALUES TO REFLECT YOUR LOCAL BUILD ###
export GLIB_CONFIG=/boot/home/glib-libIDL-beos/glib/bin/glib-config
mk_add_options GLIB_CONFIG=/boot/home/glib-libIDL-beos/glib/bin/glib-config

export LIBIDL_CONFIG=/boot/home/glib-libIDL-beos/libIDL/bin/libIDL-config
mk_add_options LIBIDL_CONFIG=/boot/home/glib-libIDL-beos/libIDL/bin/libIDL-config

### //Automatic updates not currently supported on BeOS. Delete this when we add this feature.
ac_add_options --disable-updater

### //Explicitly enable native BeOS threading
ac_add_options --with-bthreads

### //Build without Cairo
ac_add_options --disable-svg
ac_add_options --disable-canvas

### //Reduce build size and time by omitting test suite and IPv6 support (not needed on BeOS)
ac_add_options --disable-tests
ac_add_options --disable-ipv6

### //Required configure options
ac_add_options --enable-cpp-rtti
ac_add_options --enable-static ## STATIC BUILD
ac_add_options --disable-shared

### //Eliminate lots of relatively useless warnings during build
ac_add_options --disable-cpp-exceptions

### Optional optimizations
ac_add_options --enable-optimize='-Os'