So the good folks at Mozilla got a manual penalty from Google. But the message was as cryptic as ever,
Google has detected user-generated spam on your site. Typically, this kind of spam is found on forum pages, guestbook pages, or in user profiles.
As a result, Google has applied a manual spam action to your site.
Well that is certainly helpful and transparent. Even the US goverment gives you a pass on what your users create as long as you correct it when it's called out. But how can you correct something that is as elusive as a unicorn?
Google has ruined the original functionality of the WWW and links. The rule is no longer, build for your users, it's build for your users, and us, or we will slap your site.