Thunderbird add-ons that I've either used for years, or tested and highly suggest.
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
FoxClocks shows times around the world - or just your local time - at the bottom of your browser. It deals with daylight saving time so you don't have to.
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.
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 more items to Add-ons Manager context menu.
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!
Synchronizes selected addressbooks with other instances of thunderbird
using ftp/webdav/imap servers
Check the message contents and addresses before you send the message...
Allows for customization of the folder pane. Accounts can be rearranged and the startup folder can be chosen.
Displays icon for user agent of received mails.
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
This addon adds month tabs into Lightning calendar month view