MicroThunderbird, for Thunderbird 5.0 upto current nightlies.
This is based on LittleBird, sporting the same screenspace saving, but with images smaller in disk space, and more compression applied overall making the jar file very small. Net result is that TB responds much faster.
Note, includes full support for Lightning (1.0b5 and higher).
Download the file using the Firefox. But don't 'open' it.
Within Thunderbird, select Tools/Addons, and then 'Install...'
Find the downloaded file, and click open. Thunderbird will then install it.
Restart Thunderbird to complete the installation.
Note: Drag&drop from a file explorer window or even from the 'Install Now' link in Firefox into the Tools/Addons window also works (on Windows, Linux and Mac).
Please don't use the 'review' to report download errors. Download issues are not caused by the theme, but by one of the many AMO download mirrors.
Please don't use the review box for any bug reports. Contact me through the email address provided under the 'Support' heading. Using email is the quickest way to notify me, and generally I will try to make a fix available as quickly as possible.