Computer Configuration > Administrative Templates > Firefox > Addons
Both
HKLM\Software\Policies\Mozilla\Firefox
If this policy is enabled, add-ons are always allowed for the origins indicated unless add-on install is disabled. If a top level domain is specified (http://example.org), add-ons are allowed for all subdomains as well. If this policy is disabled or not configured, the default add-on policy is followed.