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.]
It’s not a block on these sites, it’s there to prevent porn and piracy sites from showing up in the “suggested” page that every privacy conscious user already turned off completely.