Tategaki allows vertical writing-mode on mail compose window.
Thunderbird HTML editor. This add-on adds a HTML tab to the composition window where you can see and edit the HTML source of the message.
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).
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.
Allows to convert large attachments to links automatically without 3rd party providers.
A simple button to invoke the (already existing) send later functions.
Warns if you send mail from or to a specified domain. If there are multiple accounts in your profile, you can isolate one so that you can't send mail to that domain from outside or vis versa without warning.
How many time you sent an email from your wrong email address?
This add-ons allows you assign a different color to the compose window border for each of your email addresses. So you can immediately view from what account are you writing.
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.
Mail Merge Kiemelt
Mass Mail and Personal Mail
ThunderPEC semplifica la gestione della Posta Elettronica Certificata (PEC) e delle comunicazioni PCT e FatturaPA in Thunderbird.
Adds the ability to easily insert emojis into your composed messages in Thunderbird.
True "Send Later" functionality to schedule the time for sending an email.
Add a button to the email composition toolbar, which lets you insert an ethical signature.
Toggle word wrapping and PRE element wrapping in the message composition and HTML display windows, respectively.
Customize background-color of the recipient address fields for each recipient types of To:, Cc:, Bcc:.Customize background-color of the recipient address fields for each recipient types of To:, Cc:, Bcc:.
(Re)submit multiple messages to a predefined recipient(s)
Adds support for the X-Unsent header.
This extension makes it very easy to generate and attach stamps to outgoing mail. It also automatically detects and verifies stamps on incoming emails. The MBound algorithm requires a 16MB fixed table to work, HashCash is more CPU intense.