I consider ads with flash or sound to be unacceptable, considering I - and I am certain others, as well - am on limited bandwidth. I also consider full page ads to be disgusting and far too intrusive. Those sorts of ads are the reason I can't even use YouTube, deviantART, and the likes until the middle of the night/super early morning when the connection allows unlimited downloads for a few hours.
I don't mind text ads or simple, non-animated, image-only ads. They're annoying, sure, but not really much of a security risk and aren't over-the-top annoying unless there's three or more per page. I accept those as a necessary evil for free services. When those are all that exist on a site, I have no qualms with allowing them to display when I have to jump on a browser other than Firefox and might even place consideration toward whitelisting them if the site has a clear cut, honest statement promising that they actively work to avoid adware/spyware in the ads accepted on the site. (Such things do exist, even if they're rare, and I have whitelisted before on sites with such policies.)
But full page ads, 'highlighted link' ads, noise-making or movie ads, and pop-ups are among those I consider never acceptable anywhere, be it a free or paid service. The bandwidth issues, security risks, and lack of easy way to "just not click" or not be scared to death by unexplained noise are just far too plentiful to consider them acceptable.
I'm honestly afraid, from what I've read in comments, that I may end up with spyware or a virus from any ads which might slip past in Firefox. I don't even want to see what it's like in a browser without built-in adblocking, just because my system - and I - am by no means willing or prepared to handle a malware attack. Otherwise, I would gladly go give it a look and see how bad it was.