Thunderbird add-ons in this collection I am trying out, but don't yet have enough experience with to endorse.
Allows bidirectional access to Google Calendar
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
Access to Google contacts and synchronize them with Thunderbird address books.
Google and Zimbra contact sync for Thunderbird. (TB 24 and above NOT supported)
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.
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!
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
Facilitates handling of duplicates in your address book(s). Allow to search and remove similar contacts.
Automatically fill your address books or send mass mails with e-mail addresses extracted from all your e-mails.
Toggle word wrapping and PRE element wrapping in the message composition and HTML display windows, respectively.
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