--- Disclaimer: this approach's been done on my personal Slackware box, you should consider your own operating system configuration, directory access permission, and environment variables ---
Actually it's quite simple:
1. download the latest update package (*.mar) available for your firefox from Firefox latest release,
2. rename the packages you just downloaded to 'update.mar',
mv firefox-18.0a1.en-US.linux-x86_64.complete.mar update.mar
3. move 'update.mar' to 'whereveryouinstalledfirefox/updates', so in my case:
mv update.mar /opt/firefox/updates
4. run 'updater update-dir apply-to-dir'. 'update-dir' refers to the location where you put 'update.mar', and 'apply-to-dir' simply refers to the location where 'firefox-bin' is. So in my case:
update-dir = /opt/firefox/updates
apply-dir = 'opt/firefox'.
update /opt/firefox/updates /opt/firefox
The installing window will pop, just wait,
5. restart firefox (if you're running any instance), and enjoy your updated build of firefox.
You can also read the 'update.log' in the same directory as the applied 'update.mar'.