Agreed, what would be the actual syntax/data to put in the "Extension Management" setting in the GPO? I just want to block extensions on devices, it would be great if I could whitelist a few extensions as well.

Glad to see that jhalder also wants the answer (means I'm not alone).

Just to clear up any possible misunderstanding, I pasted the following into the group policy "Extension Management" - but it doesn't seem to have any effect.

{

 "policies": {
   "ExtensionSettings": {
     "*": {
       "blocked_install_message": "Custom error message.",
       "install_sources": ["https://addons.mozilla.org/"],
       "installation_mode": "blocked"
     },
     "uBlock0@raymondhill.net": {
       "installation_mode": "allowed",
       "install_url": "https://addons.mozilla.org/firefox/downloads/latest/ublock-origin/latest.xpi"
     }
   }
 }

}

You don't need the policies part of your JSON code. That is the main part of the code if you are using a policies.json file, but since you are using Group Policy instead, it's not needed and actually causes the code to stop working.

The same for the ExtensionsSettings part. Again, since we are using Group Policy, Firefox already knows the specific policy that you are referring to, so you don't need to specify it.

So the example code inserted into the Extension Management policy should be the following:

 {
      "*": {
            "blocked_install_message": "Custom error message.",
            "install_sources": ["https://addons.mozilla.org/"],
            "installation_mode": "blocked"
      },
      "uBlock0@raymondhill.net": {
            "installation_mode": "allowed",
            "install_url": "https://addons.mozilla.org/firefox/downloads/latest/ublock-origin/latest.xpi"
      }
}

Hope this helps.

I've updated the documentation to try to make this clearer. Sorry about that.