Anything special about Firefox OS or Firefox Desktop apps that prevents the use of companies whose full-time business model is payments processing? Or is the special thing about fxpay the ability to pay with your phone bill?
The main reason Mozilla built a payments service and offered a custom library for developers vs. recommending Stripe (etc.) is because no one supported the direct operator billing we needed in our Firefox OS launch countries with a developer friendly interface.
Also, nothing on Firefox OS would stop developers from using any payment processor they wish to. It's all just web technology running on a mobile device so there are many payment providers to choose from, especially for US and European users.
Well it's open-source for one thing. And the MPL is a pretty accommodating license in that it works equally well alongside proprietary code, the GPL or more permissive MIT/BSD licenses.