Here is the list of add-ons I use to make my Mozilla Thunderbird Mail client more usefull.

New Features

• Folder Account
  Lets you associate user accounts and identities with specific folders.
• Remember Mismatched Domains
  Adds a “Don’t warn me again about this certificate for this domain” checkbox to the Domain Mismatch and Expired Certificate warning windows.
• MinimizeToTray
  Minimizes Thunderbird windows into the system tray.
• MoreFunctionsForAddressBook
  gives the possibility to import and export in vCard and vcf format;
• Sync Kolab 1.0.1
  SyncKolab reads a selected Imap folder and synchronizes it with the local address book and calendar
• WebMail
  Integrates web site based email accounts in to Mozllia Thunderbird: Yahoo, Gmail, Hotmail,…

Usability

• Dictionary Switcher for Thunderbird
  Displays the currently selected dictionary in the status bar …
• Display Mail User Agent
  Displays icon for user agent of received mails.

Calendar

• Lightning
  Adds a calendar to Thunderbird
• Provider for Google Calendar
  Allows bidirectional access to Google Calendar

Related posts:

1. Mozilla Thunderbird Crash (Solved)