Firefox 3.6.11 and Thunderbird 3.15 have been released which include security updates for several critical vulnerabilities that can be exploited to run malicious code. Users are advised to update these applications.

Full details about the updates here:

Firefox
Thunderbird

Its a good idea to set these applications to check for updates automatically. Here's how:

Firefox
Click Tools -> Options


Click Advanced -> Update and check the boxes beside Firefox and Add-ons

Thunderbird
Click Tools -> Options


Click Advanced -> Update and check the boxes beside Thunderbird and Add-ons