With Firefox Quantum, GPO for Firefox does not work anymore as it is a NPAPI plugin. This kind of plugins are no longer supported by Firefox.
One of the programs that is used to configure Firefox is the addo-on GPO for Firefox 0.9.2. You get an ADM file, load iton the Windows 2003 server, configure the GPO settings and you have a fully configured and finetuned Firefox at the client.
As an alternative you can use Firefox ADMX. But you need to load a vbscript every time before you can use the GPO Settings.
However, GPO For Firefox 0.9.2 has one disadvantage. The ADM file that is delivered, cannot be converted to an ADMX file with the ADMX Migrator. During the conversion it only throws up errors.
The question is now: how to move on?
If you have already used GPO For Firefox 0.9.2 then you can use a vbscript at ‘Convert Registry files (.reg) into ADMX/ADML files for GPO‘ made by Mariano Cosentino to make ADMX file from the registry settings, generated by GPO For Firefox 0.9.2. It only converts the settings in the registry file, so you cannot add policy settings later on. Or you will have to modify the ADMX file manually.
Alternatively, you can use Policypak for Firefox.