In general, you would use Windows GPO unless some systems on your environment can't use GPO. In that case, you would generally use policies.json for all systems so that it's consistent.

Mozilla.cfg is a totally separate thing. It can be used to adjust preferences that can't be adjusted with the GPO or policies.json file. If you are using that to adjust preferences that can be set with a GPO or policies.json file, it's usually best to move those preferences to GPO or policies.json.

On systems where you have both GPO and policies.json active, the GPO will always override policies.json, even in a case were the GPO has no policies active. You could separate computers in your network to have some run the GPO and some run the policies.json if you wanted to, by making sure you only add the Firefox GPO to certain computers or groups.

Hope this helps.

See also:

• https://support.mozilla.org/en-US/kb/customizing-firefox-using-group-policy
• https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson
• https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig
• https://github.com/mozilla/policy-templates/blob/master/README.md

Thanks you two for your help So if I understand you correct GPO always overrides policies.json

We try to have two firefox instances on one computer with different settings, so we can not transfer every setting into the GPO Because then the settings would be the same for both instances

So there is no way to configure two firefox instances on one PC using different settings if GPO is activ?

In your situation, you will probably find it best to use only policies.json, since it's possible to have different rules for different installations, if necessary.

GPO works globally for all Firefox installations because it uses the Windows Registry. The policies.json file is placed in the distribution folder and is only active for this Firefox installation. An autoconfig.cfg file is also placed in the Firefox program folder and thus also only works for this Firefox installation.

Thanks for the specification