Mozilla has released updated versions of its Firefox Web browser (version 3.6.14) and Thunderbird E-mail client (version 3.1.8), for all platforms (Windows, Mac OS X, and Linux). These are primarily security and stability updates. Further information is in the Release Notes for Firefox and Thunderbird.
Because of their security content, I recommend installing these updates as soon as you conveniently can. You can use the built-in update mechanism (Help / Check for Updates), or you can download the new version of Firefox or Thunderbird.
Update Tuesday, 1 March, 21:15 EST
The list of the specific security bugs fixed is now available for Firefox (10 fixes, 8 rated Critical), and for Thunderbird (3 fixes, 2 Critical). Note that the same vulnerabilities may have required fixing in both packages; they share a significant amount of code.