Crossover X for ThunderBird is a more polished rendition of Crossover, boasting an Aqua interface. It requires Thunderbird version 0.7 or higher.
In terms of requirements, you'll need Thunderbird 0.7 or later to use Crossover X for ThunderBird. But once you have it installed, you'll be able to enjoy all the benefits of a more efficient and visually appealing software. Give it a try and see the difference for yourself.
Version 2.6: N/A