Created using information from here. Uses a batch file to complete the steps required to build the customized installer. Create a custom firefox.cfg file, then drop the original installer from Mozilla onto the batch file. Script automatically downloads the extra files required, then created an exe which can be deployed normally or silently. Detailed instructions included in the download.
Note: This was created in 2009. I have no idea if any of this still works with newer versions of Firefox or not, so use it at your own risk.