The additional step for IE is enabling parameter in GPO: Computer Configuration\Administrative Templates\Windows Components\Internet Explorer\Automatically activate newly installed add-ons
In this case users will not view notice of the need to enable of add-in, but other unwanted add-ins will be activated automatically.
The add-in for Chrome can be activated via ADM and an ADMX templates added to GPO.
The add-in for Firefox can't be force enabled via GPO.
Parameter "Endpoint Security Web Control : Policy Category > Options > Web Control > Prevent user from uninstalling or disabling browser plug-in" completely works only for IE. Users can manually uninstall add-ins for Chrome and Firefox.