Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. Mozilla Thunderbird applications have too many extensions to try to write about them all, but I wanted to share some of the extensions that I find most useful for day-to-day email. Here are 12 valuable extensions for Thunderbird users that extend the functionality.
1. Nostalgy:
Adds keyboard shortcuts to change folder, move/copy messages, with folder name auto-completion (using only the keyboard). Same for tags. Also includes a system of rules to suggest a target folder for saving messages; re-creates Eudora’s Alt-Click; facilitates navigation in the 3-pane window; makes it easy to change between To/Bcc/Cc fields in the composer; facilitates tag-oriented operations. All that with the keyboard only.
Attachment Reminder is a simple Add-on (extension) for Mozilla Thunderbird that scans your outgoing email when you click the send button and checks it for the presence of user-defined keywords that signal that an attachment should be included. If there is no attachment even though the scan indicates there should be, it will show a message box asking you to confirm whether you want to send the email without attachments. You can choose to continue on and send it, or to cancel, and edit the email further.
3. GmailUI
Add concepts inspired by GMail to Thunderbird, like archiving mail and powerful message searching across all folders. Select a folder to be your sole “archive” folder, or choose to have an archive folder in each of your multiple accounts. Then type “Y” to send your selected message(s) to the appropriate archive folder. Type “from:fred to:tom” to see all messages from Fred to Tom in the current view. Press Ctrl+Enter to have this search apply to all folders.
4. BlunderDelay:
Thunderbird Extension: AutoSend Unsent Messages. Flushes entire outbox at regular intervals.
5. ReminderFox
ReminderFox displays and manages lists of date-based reminders and ToDo’s. ReminderFox does not seek to be a full-fledged calendar. In fact, the target audience is anybody that simply wants to remember important dates (birthdays, anniversaries, bills, etc) without having to run a whole calendar application. ReminderFox makes sure you remember all of your important dates via easy-to-use lists, alerts, and alarm notifications.
If you have duplicate messages (e.g. from merging folders or downloading messages several times) you can use this tool to find and remove them from the selected folder and its subfolder by rightclicking on a folder and choosing “Remove Duplicate Messages”.
7. SendTools
Add SendAt option to the compose window to allow sending a message on a particular date and time.
Switch the signature on/off or choose a new one from your predefined set.
Additional features include the support of fortune-cookie-files and automatic switching based on recipients.
9. URL Link:
URL Link is a small Firefox and Thunderbird extension that allows you to select a non-URL in a mail/news message or web-page, and open it in a browser window.
10. Clippings:
Saves and manages frequently-entered text for later pasting into Web page forms in Firefox or email and newsgroup messages in Thunderbird. Pasting a clipping is accomplished with just two mouse clicks, eliminating the hassle of retyping or repetitive copying and pasting.
Maximize message pane by collapsing the thread pane. You can read a message in larger area without opening a new message window.
12. MinimizeToTray
Minimizes Mozilla windows into the system tray. Also includes a right clickable tray icon menu to further load only the windows you want visible.

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Very good info, thanks for the post.
Great post! Thanks for sharing.
Very great post. Thanks for your research. I will see what I can use…