Mozilla Thunderbird

Table of Contents

Tips and tricks about Mozilla Thunderbid. Customizing the display, migrating the Thunderbird profile from Windows to GNU/Linux and so on…

Tip: Given the increasingly frequent changes to Thunderbid's interface, and its extension compatibility breaks, we strongly advise you to install the Addon Compatibility Check extension.

  • Download: thunderbird.net
    The Thunderbird Extended Support Releases (ESR) are mainstream releases, operating similarly to Mozilla Firefox ESR. They are stable for approximately one year before receiving feature updates :
  • wiki.mozilla.org/Releases
    Thunderbird approximately (i.e. not strictly) follows the ESR schedule listed there: wiki.mozilla.org/Release_Management/Calendar
  • Mozilla Thunderbird Blog – blog.thunderbird.net –:
    • Desktop Calendar: A Design Journey by jmiksic@thunderbird.net (10/08/2026 19:59)
      Hello, Thunderbird community and followers of the blog! This is Jesse from the design team. Today’s post is about our recent journey in redesigning the calendar portion of Thunderbird, and what you can expect us to deliver in the near future. The calendar is a key part of any productivity toolset, from your PC Desktop […] The post Desktop Calendar: A Design Journey appeared first on The Thunderbird Blog.
    • Mobile Progress Report: July 2026 by Danielle G. (30/07/2026 20:00)
      Thunderbird Mobile is moving forward with large steps on both platforms we support. On iOS, we’re working on the very foundation of a new app—our first built from scratch—and continuing to make progress in bringing the Thunderbird for iOS app to the App Store. For Android, it’s a matter of updating what we have in […] The post Mobile Progress Report: July 2026 appeared first on The Thunderbird Blog.
    • Thunderbird 153 “Meadow” is out now! by Brian Offredi (22/07/2026 22:45)
      As we head into the summer months, a new Extended Support Release (ESR) is in full bloom. Thunderbird 153 “Meadow” is out now, and from all of us at MZLA, the Thunderbird Council, and our global community of contributors, we can’t wait for you to try it out. “Meadow” builds on Thunderbird 140 “Eclipse,” along […] The post Thunderbird 153 “Meadow” is out now! appeared first on The Thunderbird Blog.
    • Community Office Hours: A Thundermail Update by Heather Ellsworth (14/07/2026 20:36)
      Our Community Office Hours series is all about connecting with the people behind Thunderbird and our products, and sharing the work that’s happening behind the scenes. These conversations are a chance to learn more about the direction of our projects and to hear directly from the people making it happen. In this episode, we are […] The post Community Office Hours: A Thundermail Update appeared first on The Thunderbird Blog.
    • Desktop settings research: what we learned from your feedback by Aly Blenkin (07/07/2026 19:56)
      A few weeks ago, we conducted hour-long conversations with 10 of our users to dig deep into how you manage your preferences and configurations in Thunderbird desktop. While this specific research cycle focused on the desktop experience, our ultimate goal is a holistic strategy that ensures our mobile settings feel like a natural extension of […] The post Desktop settings research: what we learned from your feedback appeared first on The Thunderbird Blog.

Customizing the interface

Automated customizations via addons

The rewriting of the Thunderbird 68's API breaks many extensions. That's why as of Thunderbird version 68, the following addons remains unsupported (outdated)

Manual customization of the interface with chrome/UserChrome.css

As of Thunderbird version 68 (also Firefox v.69), the syntax of the file chrome/UserChrome.css is simplified (no more namespace header neaded) but the css rule names may have changed. On October 31, 2019, except github.com/Aris-t2/CustomCSSforTb sample files, the new customization rules remains undocumented.

Beware that the path chrome/UserChrome.css is case sensitive! Not created by default, this path is relative to the Thunderbird profile folder1). 100% manual customization of the file chrome/UserChrome.css:

Prerequisites as of Thunderbird version 68 (also Firefox v.69): One have to enable the following configuration variable: toolkit.legacyUserProfileCustomizations.stylesheets (see this announce)

Simple example files /chrome/UserChrome.css:

All content of the Thunderbird window enlarged to 16pts: 

/* File chrome/UserChrome.css for Thunderbird 68+
 * Prerequisites as of Thunderbird version 68:
 * The following variable shall be enabled: toolkit.legacyUserProfileCustomizations.stylesheets
 */
 
/* _____ All content of the Thunderbird window enlarged to 16pts _____ */
:root {
  font-size: 16pt !important;
}

Screenshot of a Virtual Machine running Windows 10 with customized Thunderbird opened in a window : Screenshot of a Virtual Machine running Windows 10 with customized Thunderbird opened in a window (Credit: wiki.ordi49.fr under licence CC BY 4.0)

The folder panel and the email list enlarged to 16pts: 

/* File chrome/UserChrome.css for Thunderbird 68+
 * Prerequisites as of Thunderbird version 68:
 * The following variable shall be enabled: toolkit.legacyUserProfileCustomizations.stylesheets
 */
 
/* _____ The email list enlarged to 16pts: _____ */
:-moz-any(#threadTree) { 
  font-size: 16pt !important;
}
 
/* _____ The folder panel enlarged to 16pts: _____ */
:-moz-any(#folderTree) { 
  font-size: 16pt !important;
}

Migrate the Thunderbird profile from Windows to GNU/Linux and vice versa

  1. On the source machine – Thunderbird off – : Save all the contents of the profile directory except the extension folder and the files user.js and prefs.js. This backup includes but is not limited to the folders calendar-data/ (Lightning), chrome/, Attachments/, ImapMail/ (IMAP account directories), Mail/ (local folder directories, pop accounts and RSS feeds), News/ (newsgroups), WebmailData/ (Webmail extension), and the .dat, .db, .json, .mab, and .sqlite files. Do not backup the files user.js and prefs.js
  2. On the destination machine:
    1. Start then close Thunderbird
    2. Browse the folder of the newly created Thunderbird profile, then restore previously backed up directories and files to it
    3. Start Thunderbird, install the desired extensions, then test the send and receive
  3. If additional manual customizations have been made in the user.js and prefs.js files of the original profile, edit these files on the original machine and then copy the concerned lines into the user.js and prefs.js files of the destination profile, avoiding any reference to an extension of the original machine and transposing any possible path so that it is valid in the destination profile

Reset the main password

  • Delete (or rename to .old) the key3.db file from the Thunderbird profile folder

User-agent string

Display the user-agent of received messages

  • In the config editor, enable then variable mailnews.headers.showUserAgent to True, then restart Thunderbird. The user-agent of each email received will be displayed in the header plane
  • Other older method: addons "Display Mail User Agent" from Ernst Jürgen's website

Configuring your own user-agent

  • Within the file prefs.js of the profile folder, add : user_pref("mail.compose.other.header", "my_user-agent");

Some Thunderbird extensions

I list on this page some Thunderbird extensions divided into different categories. Each Thunderbird extension is classified into one of the following 5 levels, depending on the level of use of the software:

Level Meaning
0 essential
1 recommended (usefully complements the basic functions of the software)
2 optimizations for the familiar user of Mozilla Thunderbird
3 for users with advanced computer experience
4 for developers only
Name Level Comment
CompactHeader 1 Add symbol in header plane to switch between compact and expanded view
Auto Zoomer 1 Keep a preset zoom level across thunderbird restarts
MoreFunctionsForAddressBook 3 Adds many features to the addressbook.
(this old addons may become broken soon. See this blog comment)

Comments

1)
Reaching the profile folder: From the Help menu, click Troubleshooting Information. In the Application Basics section, Profile Folder, click on Open Folder.