Probably none, most people will probably not care and just deal with loosing some of there browsers extensions (it's not just ad blockers that will be affected) or if they care about loosing ad blockers, they'll switch to Brave without considering Firefox or any other non-Chromium browsers.
Not necessarily. But the way around it means way more dev time. I know Vivaldi and Brave have stated that they will make sure to defeat it. I assume the DuckDuckGo browser would as well but I'm not sure.
I imagine fewer people than we’d assume even install 1 extension (and half the people who do only install some shopping extension that collects all their data in exchange for price comparisons or whatever).
A millionth percent. People are not remotely aware of the whole Manifest v3 drama and will either not care for the degraded experience or put the blame on extensions developers.