Dark-colored minimalistic theme with emphasis on visibility, usability and maximum screen real estate.
It is a Theme of the Mac Leopard-style which did iLeopard in a model.
A refreshingly clean, compact theme combination for both Mozilla Firefox and its companion mailnews client, Thunderbird.
NOTE: Earlier versions and theme customization instructions are available on my site.
Looks like some Mac application.
2nd Classic is Simple and Clean Theme for Thunderbird 2.