I'll guess that it's not AIM, but perhaps an advertising aggregator that AIM uses.
There have been recent cases where malware has been sneaked into an advertising aggregator, and all the sites using it turned into a vector for malware.
The sites are actually innocent, and the advertising aggregator is only guilty in now vetting the sneaky advertiser more carefully.
However, I'll throw in a recommendation for the AdBlock extension. It's available on both Chrome and Firefox. You won't see ads from the aggregators at all -- malicious, benign, or just annoying.
Ah, I see. I have Adblock and have never seen these warnings. I just went directly to the aim.org site and didn’t get one there either.
Which specific Ad blocker extension are you referring to? There's something on the order of 300+ available for Firefox that have similar names.
I have Adblock Plus installed and still had an issue accessing FR with it.
I also use Ghostery in both Chrome and Firefox which may have also blocked access. Not 100% sure.