[Last Updated: June 2, 2008]

With the upcoming release of Thunderbird 2.0 just around the corner, I figured there would never be a better time to compile some of the best add-ons available for this popular open source e-mail client.

What is Thunderbird? Thunderbird is a free desktop e-mail and news client from Mozilla that is a popular alternative to using Microsoft Outlook. Between the software itself, and the extensions and themes that will be covered below, you can get pretty much any feature that you’ve come to rely on in Outlook, making it easy for Outlook users to make the conversion.

Does Thunderbird have Shortcuts? If you’re looking for Thunderbird shortcuts, you can find a virtually complete list of them on Leslie Franke’s Thunderbird Cheat Sheet.

How do I Install the Extensions? For experienced Firefox users, you’ll quickly learn that installing extensions on Thunderbird works a little differently. There are actually two ways to get your extension successfully installed:

Download and Install Method:

• Browse to the web page that has the download link for the extension.
• Firefox Users – Right click on “Install Now for Windows” (or whatever the download link is called) . This avoids Firefox thinking its a Firefox extension.
• Other Browser Users – Left click as normal.
• Select “Save link as” from the context menu. Press the Save button.
• Start Thunderbird.
• Press the Install button in Tools -> Add-ons -> Extensions in Thunderbird.
• Browse to the .xpi file. Press the Okay button.
• Thunderbird will display a “Software Installation” window. Press the “Install now” button when it becomes enabled.

Drag and drop to the extensions/add-ons window:

• Browse to the web page that has the download link for the extension.
• Start Thunderbird.
• Go to Tools -> Extensions (its Tools -> Add-ons -> Extensions in 2.0)
• Left click on “Install Now for Windows” (or whatever the download link is called) in your browser and drag it over the Thunderbird window in the Windows Taskbar (at the bottom of the screen). Don’t release the button. After a second the windows focus should shift to the Extensions/Add-ons window. Drag the mouse to that window and release the button.
• Thunderbird will display a “Software Installation” window. Press the “Install now” button when it becomes enabled.

That should covered everything you need, so without further delay, here are some of my favorite Thunderbird extensions currently available:

- Appearance -

• Buttons – Adds buttons to the Thunderbird Toolbar to make navigation easier.
• Contacts Sidebar - Adds your address book contacts to their own side panel.
• Cute Menus – Adds icons to your menu items.
• Flatstyle – Gives Thunderbird a Microsoft Outlook style appearance.
• Tiny Menu – Compacts your menu to save space.

- Calendar -

• Lightning – 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. (You can now sync with Google Calendar as well). Lightning Multiweek adds a multi-week view option within Lightning.

- Composing/Replying to an E-mail -

• Automatic Zip Attachments – Zips all attachments prior to sending an e-mail.
• Nested Quote Remover – Removes nested quoting automatically when you reply to an e-mail.
• Quick Quote – Allows you to quickly quote a portion of an e-mail in your response.
• Quick Reply – Adds a text box beneath the message allowing you to do a quick reply. (May not yet work with Thunderbird 2.0)
• QuickText – Allows you to insert pre-defined snippets of text into e-mails as often as you want.
• Signature Switch – Allows you to switch between multiple e-mail signatures.

- Internet -

• ThunderBrowse – Adds a URL bar allowing you to view e-mail links on the internet within Thunderbird.

- Miscellaneous -

• Del.icio.us Complete – Social bookmarks manager that integrates Del.icio.us into Thunderbird.
• Dictionary Search – Adds a dictionary to the right-click context menu.
• Email Notification Randomizer – Gives a random sound to notify you of new e-mail.
• Minimize to Tray – Allows you to minimize Thunderbird to the system tray. Can also close to system tray and more!
• Skype Toolbar – Adds a Skype toolbar to Thunderbird allowing you to call all your Thunderbird contacts via Skype.
• Update Notifier – Lets you know when there is an update to your Thunderbird extensions.
• WorldWeather – Displays your local weather in Thunderbird.

- POP3 -

• Mail Redirect – Allows you to send messages from Thunderbird to another E-mail address.
• Webmail – Allows you to get around POP3 restrictions with Yahoo and Hotmail. Installation instructions can be found here.

- Shortcuts -

• Mouse Gestures – Adds mouse gestures.
• QuickMove – Allows you to assign keyboard shortcuts to quickly move messages to assigned folders.

- Sorting -

• 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.
• GMailUI – Adds GMail style archiving and message search functionalities.
• Nostalgy – Allows for quick and easy folder navigation and message movement. Also allows you to set rules and provides several keyboard shortcuts.
• Quote Collapse – Allows you to collapse all quotes in your e-mails to make them more readable.
• Remove Duplicate Messages – Detects and deletes duplicate messages.

- Themes -

• Modern Modoki – Based on the extremely popular Fusion theme for Netscape 8.0.
• Noia 2.0 eXtreme – Silver theme with nice icons. Is based on the popular Firefox theme with the same name.
• Own – Silver theme designed to integrate well with the look of Windows Vista/Microsoft Office 2007.

As with my other lists in my “Best of” series, I will be updating this list regularly in hopes of keeping it current and useful. If you rely on any extensions or themes that I missed, please comment below with a link to the extension so I can get it added.