I don't think we'll be paying for the basic version of Firefox. Perhaps they'll release other editions and charge for those. Perhaps they won't charge consumers for anything, and only target business to business relationships. Ever notice, for example, when you install Firefox the default home page is
this? I doubt the Foundation is charging Google for that free advertising and free hits (yes, I'm aware Google donated to the Foundation so maybe that example is a wash).
Bottom line: who knows what the future holds, but I'm not concerned.