Thunderbird add-ons that I've either used for years, or tested and highly suggest.
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.
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)
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.
Extracts all attachments from selected messages and then can delete, detach or mark-read.
Reinstates the extensions.checkCompatibility preference without respect to the current application version.
Add more items to Add-ons Manager context menu.
Synchronizes selected addressbooks with other instances of thunderbird
using ftp/webdav/imap servers
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!
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.
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
Displays icon for user agent of received mails.
This addon adds month tabs into Lightning calendar month view