You can't yet. "The Firefox OS Marketplace does not currently support paid apps or in-app payments. We hope to enable this feature for testing late in the first quarter of 2013." (https://developer.mozilla.org/en-US/docs/Apps/Submitting_an_...)

The apps that I have experience with have all been no-cost.

I don't know what an IAP is, so I can't comment. :-)

edit: Thanks to Samuel for the explanation. No, I don't have experience with IAP.