If anyone's interested, I wrote a Perl script that downloads and installs Firefox Nightly on a Unix-like operating system.
By default, it installs the application into
/opt/firefox-nightly, with a link to run it at
/usr/bin/firefox-nightly. This way, it doesn't conflict with your already-installed version of Firefox. Furthermore, it will put a launcher item in your Applications/Internet menu.
You can get it from here: http://sh.kirsle.net/ffnightly
$ mkdir ~/bin $ wget http://sh.kirsle.net/ffnightly -O bin/ffnightly $ chmod +x bin/ffnightly $ ffnightly