Here is a tcsh script for building mozilla and saving all the build output to a file.  In this script "~/mozroot" should be replaced by the path to you mozilla root, and "~/mozroot/buildlog.txt" should be replaced by the filename for your build log.

#! /bin/tcsh
echo "starting build" > ~/mozroot/buildlog.txt
date >> ~/mozroot/buildlog.txt
cd ~/mozroot
cvs co mozilla/client.mk >>& ~/mozroot/buildlog.txt
cd mozilla
make -w -f client.mk >>& ~/mozroot/buildlog.txt
echo "finished build" >> ~/mozroot/buildlog.txt
date >> ~/mozroot/buildlog.txt

Why tcsh?  Well, it's the default on macosx and most folks building mozilla probably have some build cruft in their .tcshrc, like:

source /sw/bin/init.csh
setenv CVSROOT :pserver:anonymous@cvs-mirror.mozilla.org:/cvsroot
 
You can use the above script along with Cronnix to set up an automated daily build on your mac.