Have you ever accidentally sent an external email because Thunderbird's auto-complete used a different address than you were expecting? Worry no more. External Email Alert will give you a popup when you click "Send" if your are sending an email to an email address that doesn't match the domain on your email account.
2. Provider for google calendar
This extension allows Sunbird and Lightning to read and write events to a Google Calendar.
3. Xnote
Add Post-It to specific emails. Perfect to add information to an email and help you to GTD.
4. Seek
“Seek” adds faceted browsing features to Mozilla Thunderbird and lets you search through your email more effectively.
5. Auto Zip Attachments for Thunderbird
Auto Zip Attachments allows you to easily compress (zip) attachments. A new button named "AutoZip" is added to the compose toolbar. Upon pressing the button all the attachments would be compressed into a single file. Then the attachments are replaced with the zipped file.