I reckon that blocking ad blockers isn't some extra countermeasure here. It's actually right in line with what Manifest V3 and that new environment attestation system are all about. They're basically making sure that if you tinker with crucial bits of the JavaScript -- stuff they see as essential (like anti-adblock) -- you won't make it through the attestation and you'll get blocked.
They don't want to block all modifications because that would be a hindrance to many users, for example the visually impaired. However, anything affecting their bottom line will probably be blocked.
How that will affect Firefox? I don't know, maybe nothing will change for us, or perhaps Google will block Firefox altogether. We certainly know they're capable.