Sadly, I don’t know enough about it to give you advice. Every time I switched phones or services, I had to twaddle with the settings until I could get features (commonly MMS, or SMS with media) so that they worked properly. If AT&T is actually blocking you out for refusing to use an AT&T phone, the trick would be to get the phone to pretend it’s an AT&T phone, then way Firefox can pretend it’s Chrome when it needs to.

But I don’t know the specifics.