That would force them to make ABP a proxy server
While I do believe that ad-blocking will ultimately be done by a proxy server and have said this before, ABP as software as it is now is dependent on the browser's add-on architecture. The company/developer behind it could, of course, evolve both their core product and release something entirely different such as a ad-filtering proxy [service]. As for Mozilla and Firefox, yes, people will jump ship if it loses support for ad-blocking tech. If Mozilla were to cave, I'd expect the other major browsers to do so as well. Before a suggestion is made that people can always use a ad-block supporting browser growing from a fork, Jane and Joe Mainstream isn't even going to know that this is an option, let alone going to do it.