I don't have any evidence, but a hypothesis that 1% of Firefox users care enough to keep it afloat looks more realistic than if it turned out that 100% of users

I don't have any evidence, but a hypothesis that 1% of Firefox users care enough to keep it afloat looks more realistic than if it turned out that 100% of users would need to shell out $30/mo. Numbers of voluntarily paying users in open-source projects are usually pretty low, so 1% looks like the right ballpark, and is worth further research, as opposed to rejecting the approach outright as unrealistic. I'm speaking about the existing user base, who do not need converting. Also, you speak of browsers as of indistinguishable commodity goods; they are not. The pressure that Google exerts to slowly strangle ad-blocking extensions in Chrome will likely make more people consider alternatives.