It's shameful that carriers use an open system to gain more control on the user. They're the ones that are going to bill you for everything you do/buy with your phone.
Not quite sure what you're getting at here... if you use data in excess of your data plan, or you're roaming, then sure, they'll bill you for that. What phone you happen to use has no effect on this.
If you're buying software or products via your phone, then unless you're insane, you'll be connecting to the vendor with an encrypted connection, hence your carrier DOESN'T EVEN KNOW you're making a purchase.
Firefox OS uses the Android Linux kernel since drivers exist for it. There's no Dalvik, Java, or Android app.
The simplified OS helps for openness, for running on low-spec phones, and for updates & security.