Thunderbird add-ons that I've either used for years, or tested and highly suggest.
Displays the address books in a sidebar in Thunderbirds 3-pane-window and can be toggled with the F4 key or a toolbar button.
Visit my Contacts Sidebar site for recent versions and news.
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
Reinstates the extensions.checkCompatibility preference without respect to the current application version.
Extracts all attachments from selected messages and then can delete, detach or mark-read.
Add symbol in header plane to switch between compact and expanded view. Options to switch between one or two lines compact view, which buttons should be displayed in header pane. For Thunderbird 36 and older use version 2.0.9 (see version history)
The "Swiss army knife" of folders. Turn your favorites into tabs - open folders and subfolders, move mails without scrolling around or searching. powerful + easy.
See new mail at a glance after automatic filtering - supports quickFilters addon!
Add more items to Add-ons Manager context menu.
Synchronizes selected addressbooks with other instances of thunderbird
using ftp/webdav/imap servers
Allows for customization of the folder pane. Accounts can be rearranged and the startup folder can be chosen.
Check the message contents and addresses before you send the message...
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
Displays icon for user agent of received mails.
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.