Here is the procedure to follow:
- Upgrade Thunderbird and/or Sunbird if you need to.
- Install new BirdieSync version
- Launch Thunderbird or Sunbird
- Check in Thunderbird or Sunbird add-on list (menu "Add-ons / add-ons" or "Tools/Add-ons"), than BirdieSync add-on is enabled and marked as compatible
In recent versions of Thunderbird, it may happen that right after the update, the old cached version of BirdieSync is still loaded. Restart Thunderbird so that it uses the most recent version.