It seems to me that this letter seems to care about everyone ("ad-supported small businesses", "advertisers", "website owners", "Ad networks", "small publishers") except for the one who matters the most: The User. I'm not going to say what Mozilla should or shouldn't do in this case. They know better, and more importantly, they are free to do what they want. I just want to point out to IAB, whoever they are, that if…
Every time I read about another ad network vector for malicious behavior, my current position is reinforced. I feel some guilt about this, with respect to sites I like, but it doesn't override my need for "safe computing".
So... if you want your advertising to get onto my system, improve your fucking product!
Back when "the Web was young", I permitted advertising, and I actually encountered a number of interesting and useful products via it. That was also a time when the "Web" community was a lot smaller, and particularly technology-oriented advertising has a significantly higher chance of being on-target with respect at least to me as audience.
Then ads starting moving, shouting, and playing bad sets. And then came the malware. That was it, for me.
Clean up your act, and I might let you back.
Sooner or later, someone is going to figure this out.