Email client originally developed by the Mozilla Foundation and now maintained by its community. (more info)
Select a component to see open bugs in that component:
Problems with setting up multiple mail and news accounts within a single profile in Mozilla Thunderbird.
Problems with using the Thunderbird Address Book such as creating new cards, manipulating the data from the Address Book.
For bugs that result from Mozilla build problems. ("Building" is the process of converting the thousands of Mozilla source code files into a actual executable software.) If a file in a newly-downloaded build is missing or corrupted, file the bug in this component.

Examples of appropriate bugs: DLL files are missing from the ZIP file; Build contains a zero K executable.

Accessibility compliance bugs in Thunderbird: screen readers, magnifier tools support, user interface bugs affecting on disabled people experience.
Issues related to Filelink, sending big attachment, receiving etc ...
Problems with using Tools | Filters, Spam or Junk processing (aka bayesian filtering). For example: 1. Problems with creating, editing, deleting, using filters. 2. Incoming mail is not filtered properly.
The list of folders and/or the list of messages in the 3-pane window.
A "catch all" component for problems found when using the Thunderbird e-mail client which do not fall into one of the current components. Example: launch/exit crashes, features missing, etc. PLEASE NOTE!! Try to find a component which is specific to your bug first - BEFORE assigning to this component!
Inaccurate information and typos in Thunderbird's help documentation.
Bugs and feature requests for the Thunderbird application install wizard.
Issues related to Instant Messaging : Facebook, xmpp, irc, chat, etc.
Problems with main mail window, standalone message window, sidebar contents, toolbars, dialogs, etc. A few examples: 1. User interface and functionality problems within the 3-pane mail window and the standalone message window.
Problems with the writing a new email or replying to one including using the autocomplete feature.
The area that displays the header and body of the message, either in the 3-pane window or the standalone message window.
For bugs related to the new migration code for auto migration from other mail clients to Thunderbird.
Areas where Thunderbird integrates with the host desktop environment. Please use the relevant UI component for any Theme-related issues. Please use the "Core" product for other OS integration problems.
Bugs and feature requests for Thunderbird preferences. This includes the preferences manager as well as the preferences defaults.
Problems with message search capabilities in Thunderbird, including quick search widget and faceted search.
For app-level security bugs.

If the problem relates to underlying components (PSM, NSS, Core, Toolkit) then please file it in the appropriate product instead of here.

Bugs relating to the testing infrastructure that is specific to Thunderbird, e.g. Mozmill.
Bugs and requests for enhancements or changes to the default theme that ships with Thunderbird (use the OS target to indicate Windows, Mac, or Linux theme).
Toolbars and tabs in either the 3-pane window or the standalone message window.
For newly filed bugs that need some help.