In addition to releasing the new version 20 of Firefox, the Mozilla organization has released a new version, 17.0.5, of the Thunderbird E-mail client for Windows, Linux, and Mac OS X. The new version includes fixes for eight identified security vulnerabilities, threeof which Mozilla rates as critical. (These vulnerabilities also affected Firefox, and were fixed in release 20; the two applications share a considerable amount of code.) This version also includes a bug fix for a font size adjustment issue. More details are available in the Release Notes.
Because of the security fixes incorporated in this release, I recommend that you update your systems as soon as you conveniently can. You can use the update mechanism built into the software (Help / About Thunderbird / Check for Updates), or you can get a complete installation package, in a variety of languages, from the Thunderbird download page.