Firefox 3.0.1 the first security update for Firefox 3 has been released. See Firefox 3.0.1 Release Notes for more details. Current Firefox 3 users may obtain Firefox 3.0.1 manually via the Check for Updates… option from the Help menu. Automatic upgrade (for those who have this feature enabled) should occur within the next 24-48 hours. Alternatively, users may also download Firefox 3.0.1 directly by going to http://getfirefox.com.
News Source: Mozilla Developers News