Mozilla Thunderbird has lots of great features including search, customizable views, IMAP, RSS support and more.
Contacts Sidebar – displays the address books in Thunderbird sidebar. This lets you easily access all your contacts from the main window, quickly edit contact details, drop files on top of a contact to send it as an attachment and more.
GMailUI – is an extension to Thunderbird which add concepts inspired by GMail to Thunderbird, like archiving mail and powerful message searching across all folders. a folder to be your sole “archive” folder, or choose to have an archive folder in each of your multiple accounts.
Quicktext – is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails. Quicktext is the perfect tool to help you quickly answer routine, repetitive emails. The program can insert different signatures into your emails depending on whom you are sending the email to.
Signature Switch– allows you to create multiple email signatures (Private, Business, etc.) Sending emails to different people mean to use different signatures. Try save signatures and select them when you need . Also provides configurable signature auto-switch option based on the email recipients.
Auto Zip Attachments– adds an AutoZip button to Thunderbird which adds all attachments to an compressed archive, removes the files from the list of attachments and adds the compressed file instead to it. The AutoZip icon will be visible in every compose mail window in Thunderbird. You may override the default behavior of the extension and customize it to suit your needs.
Nostalgy – adds keyboard shortcuts to change folder, move/copy messages, add attachment, scroll the message while keeping the focus on the thread pane and lots more. 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.
Remove Duplicate Messages – detects and deletes duplicate messages from the selected folders .
Mouse Gestures – Allows you to execute common commands (as page forward/backward, close tab, new tab) by mouse gestures drawn over the current webpage, without reaching for the toolbar or the keyboard…
Attachment Extractor – integrates one-click attachment extract option from selected messages. Once attachments have been extracted it can delete, detach or mark messages as “Read”.
QuickFolders – allows you to bookmark easily mail folders in Thunderbird. When you drag an e-mail folder to it, you get a shortcut to that mail folder. You can also drag messages to this shortcut to move/copy them to the folder. This is handy when you have a large tree of folders for clients or projects, but use only a few of them at the same time.
Send Later – lets you schedule the delivery of emails composed in Thunderbird. It can be a contract cancellation notice, a ‘Happy Birthday’ or a friendly ‘Don’t Forget’ note.
Lighting – The extremely popular calendar for Thunderbird, modeled after Mozilla’s stand-alone Sunbird. This is the only extension truly needed to get the full feel of Outlook. Lightning Multiweek adds a multi-week view option within Lightning
Text size adds text size (increase/decrease) buttons to the main toolbar.
Mail Redirect – allows you to export messages out of Thunderbird to another email address without forwarding. It preserves all message headers and “bounces” them to a provided email address. (Thunderbird: 0.7 – 2.0.0.*)
Buttons – Adds new buttons (such as “Forward!”, “Reply!”, and “Reply All “)to the Thunderbird Toolbar to make navigation easier.
E-mail notification Randomizer – Every time when an email is received, the configured email notification sound is updated from the set of wav files in the configured folder. This extension also adds additional options in the Sound Notification preferences pane to help the user control the modified behavior.
Dictionary Search – provides a fast way to translate any word within an email message. Simply select the word you want to lookup, access right-click menu and select “Dictionary Search for .”. A new tab or browser window will open with the meaning of the word you selected.
Image Zoom– if you have an image that was too large to fit on your screen or too small to see the finer detail in your browser or email, then image zoom gives you complete control of the size of most images displayed.
Allow HTML Temp – allows to have HTML temp enabled/allowed in the selected message by only one click.
XNote – provides an option to add sticky-like notes/memos to desired messages
TimeStamp – inserts the current date/time into a message or an input field in the browser.
Skype Toolbar – initiates calls, chat sessions, transfer files to your Skype contacts within Thunderbird, view sender’s account status (online, offline), etc.
View Headers –adds a toolbar button that allows the easy toggling of viewing all message headers.
Spamato – is a spam filter extension for the Mozilla Thunderbird email client. It contains several different spam filters bundled as one powerful component which (as we believe) is much more effective than the built-in Thunderbird solution.
Smile Insert – adds smile faces to your messages.