Considering that it's based on Mozilla Thunderbird, it was a bit of a surprise that add-ons weren't available for Postbox when it debuted. That's now been remedied in Postbox beta 13 for Windows and Mac. Given Postbox's emphasis on social-networking technology and Mozilla's own success with add-ons, this move puts the e-mail client in an excellent position to attract more users.
A Webware 100 winner, the list of supported extensions isn't long at the moment, and notably it doesn't include Thunderbird's calendaring tool Lightning. Since Postbox doesn't have its own supported calendar, this could prove to be a deal breaker for some. However, the list does include several plug-ins that Thunderbird users should be familiar with, including ReminderFox, QuickText, and MinimizeToTray. MozBackup and Zindus are listed as "coming soon." There's new support for localized dictionaries from Mozilla, too.
Users who wish to install Postbox add-ons while running Firefox can either save the XPI file to their desktop and then install it manually, or drag-and-drop it into Postbox's open Add-ons window.
Other changes include fetching profile pictures from your address books in Postbox, Mac OS X, Twitter, and Facebook for the Inspector Pane. Settings can be imported from Mail.app. Multiple attachments can be dragged to your desktop. Along with a large number of stability and usability fixes, the security improvements made to Firefox 3.0.10 have also been folded in. Full release notes can be read here.