Thunderbird add-ons that I've either used for years, or tested and highly suggest.
This addon adds month tabs into Lightning calendar month view
Note from Mozilla: This add-on has been discontinued. Try MinimizeToTray Revived instead.
Minimize applications to the system tray.
Associates user-defined colors with accounts and identities. Applies these colors to accounts, identities, folders, message list, message headers and compose headers. Users will always know which accounts/identities they are working with.
This add allow to customize the snooze delay when snooze popup appears (lightning required).
Adds a summary of mail accounts and folders to replace the account central pane and the empty folder preview
(Formerly Local Install) The primary goal of this extension is to provide the tools needed to install and manage extensions and themes locally and tons of other features.
Note: With Toolkit 6.0 Auto Archiving is fully working in Firefox 2.0 and temp...
The main goal allows users to open (and in some cases edit) page source with external applications.
Latest version is always available at http://dafizilla.sourceforge.net/viewsourcewith
Adds many new mail filter actions - launch a file, suppress notification, remove star or tag, mark replied or unread, copy as "read", append text to subject.
Zoom in, zoom out, and reset zoom with one toolbar button. Works in the Compose window too.
Maximize message pane by collapsing the thread pane. You can read a message in larger area without opening a new message window...
Adds a new action to mail filters to allow playing a particular sound when the filter matches. Also includes several sample sound files.