You'll want to look at this site:
Plugin for Hosting ActiveX Controls[
^]. The site also has some interesting links and also to the source code for that plugin. I'll bet bet this will be a valuable source of information for you.
Modified:
This one's better:
http://code.google.com/p/ff-activex-host/[
^]. The site sports a links to the source and full documentation.
I think the second link is better as it specifically targets FireFox and the information on the first link seems a bit outdated.
Regards,
Manfred