This is a firefox plugin that I was looking for since long time, you will probably say why we need activeX with Firefox ? But some Intranet application require some functionnalities that are already available as ActiveX. I had a ActiveX twain solution that I wanted to run on firefox, so finally I think this could be done using ff-activex-host.

The extension is developed by Leeor Aharon, IT Structures Ltd, makes possible to use ActiveX controls in Firefox and provides full access to the hosted control (events, functions, properties) based on the Gecko NPAPI. Concerning security :

The plugin has some security related features to limit the risk it might pose to users by making ActiveX controls available in Firefox. First of all, it is using a special MIME Type so that it won't get triggered by sites that were not specifically designed for it. Additionally, it supports lists of well known CLSIDs and PROGIDs so that it can be limited to use with specific controls and interfaces. Finally, it can be "site locked" to make sure it's only being used by a predetermined list of domains.

You can find sample usage and download in the project website at http://code.google.com/p/ff-activex-host/