Why?
So basically whether Mozilla is doing the right thing or not here is entirely dependent on who the archive.is operators decide to target?
What about all the services that will be fixed for users after Mozilla makes this change, due to poorly operated DNS from the provider?
By default Firefox will fall back to the network resolver if DoH can't get the results, so the only way that a situation like this could happen is if someone purposely sabotages the DoH results like with archive.is.
Furthermore, what you are saying could basically be used to rationalize putting any kind of potentially breaking change behind an off-by-default configurable. Do you think the web would be the sophisticated application platform it is today if browser vendors actually had that philosophy? Would that actually be better for John Doe, to make them have to learn about the technical aspects of every new web technology before they are able to take advantage of them?
For example, take your average John Doe who uses Firefox. Not particularly technically competent. A new version of Firefox comes out, and all the Archive.is domains break. Who does he blame for that, and how does he solve the problem?
What's happening behind the scenes is that Firefox switched his DNS address on him without warning. And Cloudflare (the DNS endpoint used by Firefox by default) returns incorrect IP addresses for the Archive.is domains (because the admin of these domains returns fake addresses to Cloudflare from their authoritative DNS server).
Most people are using DNS provided by their ISP, so they haven't seen this problem before. I know about the problem, and my resolver (Unbound) is set to use Cloudflare over TLS for most requests, but sends Archive.is domains to Google's DNS instead. This solves the problem for me. Firefox switching to Cloudflare by default not only breaks sites like this for the average user, it even breaks my workaround that fixes the problem.
(I can't currently reproduce the problem, so maybe Archive.is caved and started sending working IP addresses. But it's the sort of problem that can happen when you start messing around with DNS. Your users will blame you if a site doesn't load in your browser, but works in other ones.)