Re: zero cost
It has all of those things.
"a 'market' implies":
"a forum for trade,": The set of browsers and the set of browser users, and the users pick their browsers.
"a benefit passing between people,": Yes, I get a browser, Mozilla gets another user which contributes to their licensing fees from Google and someone they can advertise Mozilla VPN and other commercial products to, even if I have ignored them when they do.
"a benefit that can then be traded on in other contacts of kind/money.": They can trade the money they got from Google because their user count was X for other things, and I can benefit from my having a browser. I am not required to be able to trade my browser for it to be a market. If, for example, you buy a haircut, you cannot trade it to someone else but you have still received value.
"I could go next door and get a better deal": Yes. While you aren't paying in any case, you can get one that has more control over your data, or you could stick with the Google version that prevents you from doing so. Better deals are available. My opinion on the best one may differ from yours.
But whether you understand or agree with these distinctions or not, it's still a mostly irrelevant point. Whether it's a market or not, it's still something that can be divided based on how many users each thing has, and that's what we were talking about. I think you're incorrect about this not being a market, but even if I assume you are right, what difference does it make to anything?