Group policies to install.
See Mozilla's documentation for a list of available options.
This can be used to install extensions declaratively! Check out the documentation of the ExtensionSettings policy for details.
When this option is in use, Firefox will inform you that "your browser is managed by your organisation". That message appears because NixOS installs what you have declared here such that it cannot be overridden through the user interface. It does not mean that someone else has been given control of your browser, unless of course they also control your NixOS configuration.