Thunderbird add-ons that I've either used for years, or tested and highly suggest.
Sort and search for buttons in (any) toolbar customization dialog
No Message Pane Sort prevents accidental resorting of the message pane by disabling mouse button clicks on the column headers.
It is still possible to sort messages by ctrl-clicking the header.
Email Bookmarks are here ! ... eMark your important emails.
Do you have an email that will need to revisit/attend later on ? Would you like to keep it handy and get it with a simple click ? Bookmark your important emails with the eMarks addon.
Allows you to configure a set of WebApps to open easily as tabs within Thunderbird trying to give them all the functionality that you'd find in Firefox.
Protects you from phishing attacks by verifying the From: address domain name of emails as you read them. Sender Policy Framework (SPF) and DNS-based reputation lists are used.
Forces all/any toolbar button icons to be displayed at 38x38 pixels (instead of the default 20x20 pixels). Small icons are displayed at their default size. Works with nearly all themes.
This add-on is a button on your toolbar that lets you open web apps as Tabs in Thunderbird. Use it to load Google Mail, Google Wave, Google Voice, and other "inboxes" (or any url) as tabs within Thunderbird, making it your communications hub.
Inserts the current date/time into a message or an input field in the browser.
Visit my Timestamp site for recent versions and news.
Allows other extensions to create new account types. This extension (sometimes called SkinkGlue) should not normally be downloaded directly by users, but instead is included in bundles with other extensions, such as the extension TweeQuilla.