If one or more of your add-ons is hidden after upgrading to Firefox or Thunderbird 7, installing the Add-on Recovery Tool add-on will recover them. After installing this add-on and restarting Firefox, your tabs will be restored and this tool will remove itself.
For more information, please visit this support article.
No restart required!
How to Install in Thunderbird:
1. Right-click the link below and choose "Save Link As..." to download and save the file to your hard disk.
2. In Mozilla Thunderbird, open Add-ons from the Tools menu.
3. Click the Install button, and locate/select the file you downloaded and click "OK".
· Mozilla Thunderbird