Rated 3 out of 5 stars
I've been using this theme with Thunderbird on KDE4 for a couple of years now. I like the clean, integrated look it provides, but it's not altogether complete. For example, the message list displays no icons for read and unread messages - only for messages replied to or forwarded. All-in-all, I'd give it a four star rating.
Unfortunately, a version compatible with Thunderbird 5 is unavailable from Mozilla Add-ons. For that you'll have to go to https://github.com/merike/oxybird2/tree/master/oxybird-linux-2.1. Ignore "oxybird-linux-2.1.jar"; it's not a valid Java archive file. Instead you'll have to download the source file.
After you've unzipped the source file archive, locate the valid .jar file in sub-folder oxybird-linux-2.1. Curiously, the theme shows up as "OxyBird2 (version) 3.0" in the Add-ons Manager for Thunderbird 5. Worth the effort? Sure, but it brings my overall rating down to three stars.