Firefox 3.0.16 and Firefox 3.5.6 were already released and are available for download (link available). Firefox 3.0.16 and Firefox 3.5.6 are both regular security and stability updates. Download Firefox 3.0.16 and Firefox 3.5.6 now.
If you want to keep your Firefox browser as updated as possible, you have to do either of the following:
Open Firefox. In the Help menu, select Check for Updates.
Or in the Tools menu, go to Options > Advanced > Update > check Firefox.
Or download Firefox 3.0.16 here and Firefox 3.5.6 here.