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 –:
    • Thunderbird 2025 Review: Building Stronger for the Future by Monica Ayhens-Madon and Natalie Ivanova (18/12/2025 13:00)
      2025 was an exciting year for Thunderbird. Many improvements were shipped throughout the year, from faster updates with a new release cadence, to a modernized codebase for the desktop app. We made big strides on our mobile apps and introduced the upcoming Thunderbird Pro to the world. As we wrap up the year, a huge […] The post Thunderbird 2025 Review: Building Stronger for the Future appeared first on The Thunderbird Blog.
    • Thunderbird Monthly Development Digest: November/December 2025 by Monica Ayhens-Madon (17/12/2025 17:23)
      Hello again from the Thunderbird development team as we start to wind down for the holidays! Over the past several weeks, our sprints have been focused on delivery and consolidation to clear our plates for a fresh start in the New Year.  Following our successful in-person work-week to discuss all things protocol, we’ve brought Exchange […] The post Thunderbird Monthly Development Digest: November/December 2025 appeared first on The Thunderbird Blog.
    • VIDEO: Exchange Email Support by Monica Ayhens-Madon (11/12/2025 18:16)
      Welcome to the last Community Office Hours of 2025! In this edition, Heather and Monica welcome Sr. Software Engineer Brendan Abolivier and Software Engineer Eleanor Dicharry from the Desktop Team. We’re discussing the recent Exchange Web Services Support for email that just landed in Thunderbird Monthly Release 145. Learn how the team landed this feature […] The post VIDEO: Exchange Email Support appeared first on The Thunderbird Blog.
    • Thunderbird Send Security Audit with OSTIF and 7ASecurity by Natalie Ivanova and Monica Ayhens-Madon (09/12/2025 17:07)
      As we get ready for the Thunderbird Pro launch, we want every service we offer to be secure and worthy of the trust our community places in us. That means being honest about where we stand today and the work we are doing to meet the promises we are making. Recently we partnered with OSTIF, […] The post Thunderbird Send Security Audit with OSTIF and 7ASecurity appeared first on The Thunderbird Blog.
    • State of the Thunder 14: The 2026 Mobile Roadmap by Monica Ayhens-Madon (02/12/2025 15:04)
      Welcome back to the latest State of the Thunder. In the last Community Office Hours, Heather and Monica sat down with members of the mobile team in a retrospective to celebrate the first year of the Thunderbird for Android app. In this recording, however, Alessandro is leading viewers through the upcoming mobile roadmap, both for […] The post State of the Thunder 14: The 2026 Mobile Roadmap 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.