Can you make it work in Postbox 3 Rated 5 out of 5 stars
Works great in Thunderbird but won't work in Postbox - although it installs OK. No extra lines visible in Compose window
Sorry, but I don't use Postbox. Perhaps one of the add-on authors there can help you out. The addressing widget in Postbox may be different or they may be using the widget that was coded prior to Thunderbird v5.0.