4.1 Extensions

Extension	Source	Main Purpose	Notes
Cardbook	Source	Access radicale instance for contacts	Thunderbird v102+ got a new address book with vCard support
Header Tools Lite	Home	Used to edit References to have correct Message-ID to maintain threading on mailing lists when not subscribed. Unmaintained and incompatible with Thunderbird 68.0+, however there is an alternative solution.
Lightning	Source	Calendars	Thunderbird v78+ integrates Lightning
Markdown Here	Source	Write pretty HTML emails sometimes
Enigmail	Source	PGP	Thunderbird v78+ includes built-in encryption
DKIM Verifier	Source	Verify DKIM signatures

Alternative solution

Borrowed from MozillaZine: Custom headers. Archive link 1, 2. Just in case it disappears.

1. Check the value of: mail.identity.idX.useremail to determine which index number refers to your email account. ie Edit → Preferences → Advanced → General → Config Editor
2. Set user_pref("mail.identity.id1.headers", "References, InReplyTo"); to create the custom headers.
3. Change the custom headers, you could use a user.js for this.
4. Set the values for References, you can use as many References as you like, just make sure they are separated with a new-line character and a space ie: \n . user_pref("mail.identity.id1.header.References", "References: <[email protected]>\n <[email protected]>");
5. Set the In-Reply-To: ie user_pref("mail.identity.id1.header.InReplyTo", "In-Reply-To: <[email protected]>");
6. Restart Thunderbird and send your email.
7. Comment out the above set options in your user.js with /* */. Go back into about:config ie (Edit → Preferences → Advanced → General → Config Editor) and rightclick on the keys mail.identity.id1.headers, mail.identity.id1.header.References and mail.identity.id1.header.InReplyTo and click Reset. Do not set these values to anything else in a user.js, because it will interfere with Thunderbird's normal behavior when you click Reply on an email.

Installing extensions from AMO (ATN)

To allow smooth extensions install from AMO (https://addons.thunderbird.net/) with thunderbird user.js, you'll need to :

• re-enable JavaScript (9230, javascript.enabled) ;
• re-enable mozAddonManager API (4503, privacy.resistFingerprinting.block_mozAddonManager) ;
• disable RFP, at least for addons.thunderbird.net (4505, privacy.resistFingerprinting.exemptedDomains).

If you are not configured this way, Thunderbird will download extensions as .XCF archives instead of properly installing them.
If you don't miss AMO integration, you can always download extensions before manually opening them using Add-ons and Themes > Manage Your Extensions (cog wheel) > Install Add-on From File....