This is a guest post from Mark Banner.
With the Thunderbird 5 Beta release, we’ve enabled the 5.* maximum version in the AMO Developer tools. Thunderbird 5 Beta is now API frozen, so if you haven’t already done so, please make sure your add-on is compatible. We expect it to be released near the end of the month.
For add-ons, the most significant change in Thunderbird 5 is the new Add-on Manager that was incorporated into Firefox 4. There is documentation and further information on developer.mozilla.org.