A) Maybe not you, maybe not me or anyone else here, but 99.99% of the rest of the world? And when the rest leave, is Mozilla really going to be able to justify maintaining a browser for those that remain? B) There might not be a website that would do it, but what about if practically all websites with any corporate backing did it?

I may not be 100% right, as I haven’t looked at it in detail, but I think it’s even a bit more than that. Since the way that’s proven is by the browser vendor signing the request (I assume with an HTTP header or something), you could also verify it’s from a specific vendor. So even if Mozilla says, yes, we’ll display your ads, a website could still lock down to Chrome. It would probably also significantly hamper new browsers, and browsers with a security/anti-ad focus, as they won’t be recognised by major websites that use the new protocol until they have market share, which they won’t get if they don’t have access to major websites.