The Mozilla ActiveX Control uses the Gecko layout engine to deliver a fully programmable HTML and XML rendering control for ActiveX developers. The API is similar to the Internet Explorer ActiveX control so it maintains a high degree of compatibility.
Mozilla ActiveX Control can be used by ReactOS to gain compatibility with some Windows programs, as well as enable web browsing right from within ReactOS Explorer.
Gecko is the same engine that powers Mozilla, Mozilla Firefox and numerous other rich content applications.
References
Mozilla ActiveX Control Wikipedia(Text) CC BY-SA