Along with the Firefox 12.0 release today, Mozilla has released version 12.0 of its Thunderbird E-mail client, for Mac OS X, Linux, and Windows. The new version includes improvements to global search, and to RSS feed handling. It also fixes 13 security flaws, 6 of which Mozilla rates as Critical. (Firefox was affected by many of the same vulnerabilities; the two packages share a substantial amount of code.) More details of the changes are available in the Release Notes.
You can get the new version via the built-in update mechanism (Help / Check for Updates), or you can download versions for all platforms, in more than 50 languages. Because of the security content of this release, I recommend that you update your system fairly soon.
Update Tuesday, 24 April, 18:06 EDT
The link for the Release Notes, above, originally pointed to the notes for version 11.0; it has now been fixed to point to version 12.0. My apologies for the error.