Those are the MD5 hashes of a lot of adult sites. They list them, so they can filter them from “often visited”. In https://bugzilla.mozilla.org/show_bug.cgi?id=1159884 they reason, that this is so that virus scanners don’t detect FF as malicious (due to containing references to those sites). Let’s get the list!
You can also access this list locally by typing in resource://activity-stream/lib/FilterAdult.sys.jsm into your URL bar.
[Note that this post is technically not NSFW as they are only MD5 hashes.]
Is there a feature to block these sites that I am unaware of?
According to the linked ticket in the OP:
Problem: commercial partners do not want their content to be negatively associated with adult content. In the context of Suggested Tiles, this means no sponsored or affiliate tiles should appear within same browser viewport. As an example, MGM would not want a 007 DVD release to be appearing within the same page (in particular, directly next to) where PirateBay tile appears.
