Interestingly, it works if you request the desktop site: they're explicitly blocking Firefox Mobile.
They probably just don't want you accessing the store from a browser on mobile when you can be using their app. I don't think this is some malicious betrayal of trust, even if its a stupid thing to do.
It's annoying if in fact it's very specifically Firefox Mobile and nothing else, but I agree, if the app has the better experience, they likely want to funnel everyone that way.
Edit: aaaand down-voted. What is the actual problem here?