There are some pretty major changes coming in Firefox 21 that you need to be aware of if you are going to deploy it. Here's the details.
In order to support the metro version of Firefox, most of the Firefox code was moved into a subdirectory called browser. This means that all configuration files except the autoconfig now go into the browser subdirectory:
So just about every blog post I've ever done on this stuff doesn't apply starting with Firefox 21. Also be aware that these changes will be in the ESR 24.
In addition, there is a new feature called Firefox Health Report that is on by default. You can see it by going to about:healthreport. This feature sends data about Firefox to Mozilla. You have two choices here. If you want to be able to see the data, and just not have it sent to Mozilla, set the preference "datareporting.healthreport.uploadEnabled" to false. If you want to turn it off completely, set the preference "datareporting.healthreport.service.enabled" to false.