There's something called FF ActiveX Host at
http://code.google.com/p/ff-activex-host . It doesn't seem to be maintained, but it might work. Another possible solution is IE Tab, a Firefox Add-on (Windows only) that doesn't make ActiveX work on Firefox, but makes an IE browser window appear inside a Firefox browser. I use that when I have to use Sharepoint at work. It automatically opens those pages with IE, but otherwise still looks like I'm using Firefox. Of course, all your Firefox users would have to install that Add-on and configure it to activate whenever they went to one of your pages. Also, they'd have to be on a Windows platform.
The best thing to do is refactor your web pages to not use ActiveX, which is an outdated, insecure, and needlessly proprietary technology.