The Mozilla Foundation marketing site SpreadFirefox.com, which is designed to promote adoption of Firefox, was hacked over the weekend, officials said in an e-mail sent to users on Friday.
Attackers exploited a security vulnerability and potentially accessed personal information of the sites registered users.
The breach was not discovered until July 12, and the Web site was subsequently taken offline.
Mozilla Foundation officials believe the compromised server was only used to send spam, but opted to err on the side of caution and contact users.
“It doesnt look like the attacker accessed any personal data on the site, but to be safe, were encouraging all of our users to log in and change their passwords,” read a notice posted by Mozilla engineer Asa Dotzler.