These are the extensions that I currently use in Thunderbird.
Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
Allows bidirectional access to Google Calendar
Access to Google contacts and synchronize them with Thunderbird address books.
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.
Synchronizes selected addressbooks with other instances of thunderbird
using ftp/webdav/imap servers
Collections are groups of related add-ons that anyone can create and share.Explore Collections