Computer Configuration > Administrative Templates > Firefox > Bookmarks
Both
HKLM\Software\Policies\Mozilla\Firefox
If this policy is enabled, you can use JSON to configure bookmarks, including [] to clear all bookmarks. If this policy is disabled or not configured, the individual bookmark policies are used. If this policy is enabled along with individual bookmarks, those bookmarks will not be added. This policy has no effect on Managed Bookmarks. For detailed information on the JSON, see https://github.com/mozilla/policy-templates/blob/master/README.md#bookmarks.