That doesn't explain the apparent market share of 2--3%, which is still quite large if you think about.

I believe most non-techie users are just lingering, using Firefox just because they used to. Since Firefox doesn't have a built-in ad blocking and the knowledge about adblocking is not universal (see my other comment), it is possible that there are a large portion of Firefox users who don't use adblockers and conversely adblocking users are in a minority. If this is indeed the case, Mozilla can (technically) take such a bet as such policy will affect a smaller portion of users. But that would work only once; Mozilla doesn't have any more option like that after all. That's why I see $150M is plausible, but only once.