SmartTemplate4 offers an easy way to automatically create customized replies by auto-inserting various mail headers. Templates can be created for every account as well as for the default (all email accounts). Also supports the Stationery Add-on.
Insert smilies in messages.
True "Send Later" functionality to schedule the time for sending an email.
Transliterator No Restart
As-you-type phonetic conversion of text from one alphabet to another, as well as transliteration/detransliteration of selected text anywhere within documents. Supports multiple languages and custom transliteration schemes.
Mail Merge Featured
Mass Mail and Personal Mail
Adds a font size (in pixels), strikethough, subscript and superscript buttons to Thunderbird's Compose Window.
Automatic Dictionary No Restart
Remembers the language you use with a person or group of people, and switches the spellchecker language automatically for you next time. You do not have to do it manually any more. It remembers and switches it for you.
Vietnamese-language input method editor (IME). Type diacritics directly into any Web page, Web app, or dialog box. Convenient, comprehensive, secure.
Bộ gõ tiếng Việt cho phép đặt dấu trực tiếp vào bất kỳ trang Web, ứng dụng Web, hộp thoại.
Save frequently-entered text for pasting later.
Category manager for CardDav contacts, also allows to send an email to all members of a category (categories can be used as contact groups).
ThunderPEC semplifica la gestione della Posta Elettronica Certificata (PEC) e delle comunicazioni PCT e FatturaPA in Thunderbird.
Adds Text Color Buttons in Compose Toolbar and More: Pixel FontSize, Strikethough, Sub & Superscript, Lower & UpperCase. Also provide Tools: Insert iframe, Launch MathType, Maths & Greek Symbols, Load Template Variables and Duplicate Compose Window.
Easily and quickly to encrypt / decrypt:
Private messages via e-mail, blogs, social networks, forums, websites, images, etc
Support: Firefox, Thunderbird and SeaMonkey
New feature: CryptoPad (crypto-notepad)
Adds a menuitem to the Mail context Menu to Reply to a different Mailadress
Quickly highlight some text in Thunderbird message composing windows.
Write your email in Markdown, then make it pretty.
Adds a button to send an email and archive the original message.
Check the message contents and addresses before you send the message...
Extension to mimic Outlook Express Stationery.
Allow you to use HTML files as templates for mail. Also allow custom template in replies or forwarded mail!
Save your email into Evernote by forwarding it to Evernote email address.
Before you use it, you need to specify your Evernote email address (see the screenshot).
This function can be invoked by "Forward to Evernote" in the context menu.