There is a project that provides MSI files of Firefox:
Firefox MSI Packages
However, you can achieve near the same result with group policy and startup scripts. You can assign a script to each PC as a startup script that will install/upgrade Firefox (or any non-MSI package for that matter). When it reboots, it runs the script and installs it. Typically you would want to check the version on firefox.exe to see if it needs upgraded and if not move on without installing again. You can even make a computer group and assign the script to the group to do a targeted deployment. It does lack the management of AD deployed software, but you can work around most of it. For example, if you wanted to remove Firefox later, you would use the same method to publish a script that would uninstall it.
Example of how to install silently in a script:
Set Installation = WshShell.Exec("\\server\share\firefox-installer.exe /S /INI=""\\server\share\customizations.ini""")
Do While Installation.Status = 0
When installing, you will want to use any switches to make it silent since there won't be a user there to interact with it. As far as managing the customizations, you have the option of making custom group policy templates. There are also several resources available online that already have a few of these:
Additionally you can use a custom ini when installing to set defaults. This is the method I use personally. I like to let users choose when possible. They aren't going to be likely to change anything that is required for functionality (your authentication/proxy settings for example) so it's generally safe to trust them.
Installer:Command Line Arguments