Along with the Firefox 13.0 release earlier in the week, Mozilla has released version 13.0 of its Thunderbird E-mail client, for Mac OS X, Linux, and Windows. The new version includes a built-in capability to upload large attachments to a “cloud” service; it also has a mechanisms by which you can sign up for your own Internet domain name. It fixes seven security flaws, four 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.