The issue is we currently install the latest Firefox 24. I have nor proven this yet, but I
think the install overwrites the current config file(s) back to default, which means Firefox has to
be re-STIGed. Am I correct? Is there a proper update for Firefox rather than a fresh install?
Depending which Firefox "installer" you use, the config files that are provided by that installer can be overwrited. But this is not the case for the files that are in $HOME/.mozilla.
If you want to use an update method, you can still use a "mar" file that is available for each releases in Mozilla FTP server. The instruction are at : https://wiki.mozilla.org/Software_Update:Manually_Installing_a_MAR_file