Another big issue is that if chrome makes it difficult to disable ads such that 99% of chrome users aren't able to do it, websites may simply choose to block Firefox as it would be easy to do so without losing a large part of user base while making sure no users are blocking ads. Right now there are far too many users using ad blockers.
Blocking a user agent? Oh no, whatever shall we do? Thankfully, we can still just change it, though I suppose a really insidious web site could use feature checks.
Once one major site does it, they all will.