Mozilla has released a new version, 3.6.12, of it Firefox browser, for Mac OS X, Linux, and Windows. This update fixes a critical security vulnerability, which potentially allows a remote attacker to run arbitrary code on the target system. You can get the new version via the built-in update mechanism (Menu: Help / Check for Updates); alternatively, you can download installation packages for all platforms, in many different languages. Further information about the new version is available in the Release Notes, and specific information about the patched vulnerability is in the Security Advisory MFSA 2010-73.
Because of the seriousness of the flaw, and because there have reports of exploits for the flaw on the Internet, I recommend installing this update as soon as you can.