All of the suggested add-ons from Meryl's Web Worker Daily article.
Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
Allows bidirectional access to Google Calendar
LookOut is a plugin which allows Thunderbird to interface with Microsoft's mail tools by decoding metadata and attachments encapsulated/embedded in a TNEF encoded attachment (aka winmail.dat).
Switch the signature on/off or choose a new one from your predefined set...
Displays the address books in a sidebar in Thunderbirds 3-pane-window and can be toggled with the F4 key or a toolbar button.
Visit my Contacts Sidebar site for recent versions and news.
Extracts all attachments from selected messages and then can delete, detach or mark-read.
Check the message contents and addresses before you send the message...
Keyboard shortcuts to change folder, move/copy messages, with folder name auto-completion.
Collapse and expand quotes. Please see the extension homepage for...
A Thunderbird extension which disables drag and drop in the folder pane.
This extension prevents unintended folder movement.
Email Bookmarks are here ! ... eMark your important emails.
Do you have an email that will need to revisit/attend later on ? Would you like to keep it handy and get it with a simple click ? Bookmark your important emails with the eMarks addon.