Following its support policy, Mozilla has released updates for the Firefox and Thunderbird 1.5.x branches.  In the Help menu, select Check for Updates… and you will be prompted to proceed with the update. Remember to restart the application for the updates to take place.

For Firefox, this update brings the usual stability and security improvements, including patches for three critical vulnerabilities. Following Mozilla Support Policy, Firefox 1.5.x will be supported until April 2004, 2007 (6 months after the latest release) and users are strongly recommended to upgrade now. 

For Thunderbird, 1.5.x is the current branch and so is under full support. It brings security and stability enhancements as well, including fixes for three vulnerabilities, one labeled as critical.