In no particular order…

• Bugmail

  Thunderbird 2.0 – 3.0b2pre

  A bugtracker bugmail companion – provides a nice header with bug status, upstream links, etc. work nicely with Launchpad bug mail

• Buttons!

  Thunderbird 2.0b1 – 3.0

  Adds buttons! Not sure how to explain them all…

• Compact Menu 2

  Thunderbird 2.0 – 2.0.0.*

  Duplicates the menubar on the toolbar as a menu of menus.

• Delete Junk Context Menu

  Thunderbird 0.5 – 3.0

  Adds ‘Delete Mail Marked as Junk’ to folders’ right click menu

• Diccionario de Español/España

  Thunderbird 2.0 – 3.0a2pre

  Diccionario de Español/España

• Dictionary Switcher

  Thunderbird 2.0b2 – 3.0a1

  Displays the currently selected dictionary in the status bar and allows to change it.

• Dictionnaire MySpell en Français – part of Ubuntu

  Thunderbird 1.5 – 2.0.0.*

• Display Mail User Agent

  Thunderbird 2.0 – 3.0a2pre

  Displays the mail program with which the selected mail was written – always useful to give some context to email – is the author using Windows, Linux, a Mac ?

• Duplicate Contact Manager

  Thunderbird 1.5 – 2.0.0.*

  Facilitates handling of duplicate contacts in your address books. You know you have some.

• Enigmail – provided by Ubuntu, but I install it manually

  Thunderbird 2.0 – 2.0.0.*

  OpenPGP message encryption and authentication

• Extension List Dumper – produces this list in nice HTML

  Thunderbird 1.5 – 2.0.0.*

  Dumps a list of the installed extensions.

• Lightning – I install this one manually too.

  Thunderbird 2.0 – 2.0.0.*

  An integrated calendar for Thunderbird (build 2008091718)

• Mail Redirect – a must to save some « here’s this email I got but it should go to you  » intros.

  Thunderbird 0.7 – 2.0.0.*

  Allow to redirect (a.k.a. « bounce ») mail messages to other recipients

• Provider for Google Calendar

  Thunderbird 2.0a1 – 2.0.0.*

  Allows bidirectional access to Google Calendar

• Quicktext – great for multiple signatures and templates

  Thunderbird 1.5 – 3.0a1

  Adds a toolbar with unlimited number of text to quickly insert. It’s also possible to use varibles like [[TO=firstname]]. With settings for everything.

• QuoteCollapse – you know, some people just looooove to quote.

  Thunderbird 0.7 – 2.0.0.*

  Collapse quotes.

• Remember Mismatched Domains – yes I use Dreamhost and SSL.

  Thunderbird 1.5 – 2.0.0.*

  Adds a ‘don’t ask me again’ option to the Domain Name Mismatch Security Error window.

• Remove Duplicate Messages (Alternate) – no comment!

  Thunderbird 1.5 – 3.0a1

  Locates duplicate messages in mail folders and removes them.

• Sender Verification Extension

  Thunderbird 0.6 – 2.0.0.*

  Protects you from phishing with SPF and DomainKeys.

• Tag Toolbar

  Thunderbird 2.0b1 – 2.0.0.*

  Add toolbar for toggling tags.

• Xpunge

  Thunderbird 1.0 – 2.0.0.*

  Empties the Trash folder and compacts folders in multiple Thunderbird accounts.

If you have any that make your Ubuntu life easier, let me know!