A database containing usernames and password hashes belonging to users of addons.mozilla.org had been posted publicly by accident. Mozilla has deleted the passwords of all 44,000 accounts from the site regardless of whether they were exposed or not. The only person outside of Mozilla who accessed the content was the person who disclosed the accidental publication to them through their web bounty program. If you were one of the unlucky recipients of one of these emails, make sure you were not using the same password at Mozilla as you are at other sites. Remember, unique passwords are a requirement, not a luxury.
Source: https://thehackernews.com/2010/12/mozilla-accidentally-publishes-user-ids.html