Two new releases from Mozilla, Firefox 3.5.3, and for those still using Firefox 3 there is Firefox 3.0.14

These are both security/stability updates.
Firefox 3.5.3 Release Notes
Firefox 3.0.14 Release Notes

One new feature of the update is that it will check the version of the Adobe Flash Player that you have installed, and if it is an old version you will be advised to update the Flash Player. This is because old versions of the Flash player have security and stability issues, and are one of the main causes for Firefox crashing. In the future the plugin check may also cover other plugins.