The XYZ guys have a quick write-up on what they feel are the best extensions for Mozilla's new email client, Thunderbird 2.0. I'm personally just a bit ticked that Plaxo hasn't finished their extension for 2.0 yet >:(

"Between these changes and my general requirement of using as few extensions as possible I have found that Thunderbird does not need many extensions to get it set up properly. There are a few excellent ones out there that everyone should know about."