Can I force a link to open in a specific browser?

It really depends on your exact situation.

If you are targeting multiple users outside a corporate network this is hardly possible to achieve. However, if you are targeting your company's network it's possible to do this using custom protocol.

Your link then would look like: ie://opens_in_ie.com

You'll have to make changes to registry of target computers though. More info about custom protocols is available here: http://msdn.microsoft.com/en-us/library/aa767914%28v=vs.85%29.aspx

In most cases, system administrator can do this remotely.

I don't think you can open a IE window from firefox, but you can easily build a firefox plugin based on your activex using http://code.google.com/p/ff-activex-host/