And ad supporters say that not providing facilities for personalized ads means sites wouldn't be able to exist.

I’m not saying they wouldn’t be able to exist though. I’m saying you wouldn’t be able to use them. Firefox does not have the market share to be able to force change there, they’d only be making the experience shittier for their users. You’re welcome to turn the feature off if you don’t like it.

Ad supporters are wrong about this as a blanket statement. There are tons of useful sites that exist, many for years or decades, without any advertising at all.

Some sites would go behind paywalls, some would cease to exist, and some will just run nontargeted ads, but some would do none of those things.

There is no difference.

These sites that can't be used without DRM plugin do not provide you a way around DRM because you're ruining their business model (at least they think so, or their content providers).

The same goes with ads. If your browser start for example blocking ads at sites that live from it (like New York Times website), website administration will eventually ban your browser at all.