They've been quietly preventing Firefox from becoming a threat for a long time. There are constant little things that just mysteriously don't work as well on Firefox, for no reason. People have changed the user agent and found that it works just like on Chrome with Chrome's agent. Youtube was doing it for a while, and reviews on the search are another instance. I was at the Dentist's and they were asking for a Google review, but I couldn't find the spot to leave it. I switched to Chrome and it was magically right where it was supposed to be.

So they already think Firefox could be a threat, and preventing ad-block is going to make it a bigger threat.