everything would work just fine - ubuntuzilla package places a redirect on the ubuntu package, so that its startup link goes into /usr/bin/firefox.ubuntu. so when new version comes out in the ubuntu repos, it knows not to overwrite /usr/bin/firefox, but instead writes its link in firefox.ubuntu. so you can install those updates without fear! :)
...somewhat unrelated, but I was wondering, what would happen to /usr/bin/firefox when Firefox 10 finally hits the repositories? After installing the package from Ubuntuzilla, the link points to /opt/firefox/firefox, so wouldn't it get overwritten?