Bug 267888 has been open forever, and I really don't think anything will be done to get true group policy support in Firefox. Things like the CCK and other tools don't solve the core problem because the whole point of group policy is that when it is enabled, any version of a given product that is installed will use the policy. The problem with Firefox solutions is that they are bundled with a given version of Firefox, so any other version of Firefox can be downloaded that doesn't use the policy. My proposal is that Firefox support exactly one policy - the location of the AutoConfig file. If specified, Firefox would always use that AutoConfig file. This would allow a much simpler way to implement group policy in Firefox using the existing AutoConfig mechanism.
Moving to component where the code actually lives.
For anyone that wants to look at this, the API is GetAppliedGPOList: https://msdn.microsoft.com/en-us/library/aa373518%28v=vs.85%29.aspx For examples of Chrome code that does this, see: https://code.google.com/p/chromium/codesearch#chromium/src/components/policy/core/common/policy_loader_win.cc&q=ReadPolicyFromGPO&sq=package:chromium&type=cs&l=560