Update: It turns out that Google only penalized a single page from Mozilla. Matt Cutts weighed in on the "penalty" in that same forum thread (hat tip: Search Engine Land).
Google has penalized Mozilla.org, the nonprofit site of the organization that provides the Firefox browser. This doesn't appear to be an accident like what recently happened with Digg. This was a real manual web spam penalty.
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.
"I am unable to find any spam on http://www.mozilla.org," said More. "I have tried a site:www.mozilla.org [spam terms] and nothing is showing up on the domain. I did find a spammy page on a old version of the website, but that is 301 redirected to an archive website."
Google Webmaster Trends analyst John Mueller responded:
To some extent, we will manually remove any particularly egregious spam from our search results that we find, so some of those pages may not be directly visible in Google's web-search anymore. Looking at the whole domain, I see some pages similar to those that Pelagic (thanks!) mentioned: https://www.google.com/search?q=site:mozilla.org+cheap+payday+seo (you'll usually also find them with pharmaceutical brand-names among other terms).
In addition to the add-ons, there are a few blogs hosted on mozilla.org that appear to have little or no moderation on the comments, for example http://blog.mozilla.org/respindola/about/ looks particularly bad. For these kinds of sites, it may make sense to allow the community to help with comment moderation (eg. allow them to flag or vote-down spam), and to use the rel=nofollow link microformat to let search engines know that you don't endorse the links in those unmoderated comments.
For more tips on handling UGC (and I realize you all probably have a lot of experience in this already) are at http://support.google.com/webmasters/bin/answer.py?hl=en&answer=81749
Also keep in mind that we work to be as granular as possible with our manual actions. Personally, I think it's good to react to a message like that by looking into ways of catching and resolving the cases that get through your existing UGC infrastructure, but in this particular case, this message does not mean that your site on a whole is critically negatively affected in our search results.
Let this be a lesson to all webmasters and bloggers. Keep your comments cleaned up.
Mozilla still appears to be showing up in key search results like for "mozilla" and for "web browser". It's not as bad as when Google had to penalize its own Chrome browser for paid links.