Mozilla Thunderbird is a free and open-source program used mainly for managing email, calendars, and contacts. It also includes tools for reading news feeds, chatting online (using IRC, XMPP, or Matrix), and accessing news. Thunderbird is run by MZLA Technologies Corporation, a company owned by the Mozilla Foundation. It is a project managed by the Thunderbird Council, a group of people chosen by the Thunderbird community. Thunderbird works on many operating systems, including Windows, macOS, FreeBSD, Android, and Linux. The program was first designed using a similar plan to Mozilla's Firefox web browser and is built using that browser's technology.
Overview
Thunderbird is a tool that helps manage personal information, including email, discussion groups, news updates, instant messaging, and calendar features. Starting with version 78.0, calendar functionality was added directly to Thunderbird. Before this, users could install the Lightning calendar extension. Additional features can be added by installing special files called XPInstall modules, also known as "XPI" or "zippy" files, through the add-ons website. This website also allows users to update these added features.
Thunderbird supports different themes that change how the program looks. These themes are made up of CSS and image files and can be downloaded from the Mozilla Add-ons website.
Thunderbird has been translated into more than 65 languages by contributors worldwide. However, email addresses can only use ASCII characters for the local part of the address.
On June 13, 2022, it was announced that the Android app K-9 Mail was taken over by MZLA Technologies Corporation, a company owned by the Mozilla Foundation. The plan was to rename K-9 Mail to Thunderbird for Android, allowing it to sync with Thunderbird on computers, use Thunderbird's automated account setup system, filter messages, and improve folder management.
On October 30, 2024, the first official version of Thunderbird for Android (called Thunderbird Mobile) was released as version 8.0. On May 6, 2025, version 10 and the beta version of 11 were released. Thunderbird for Android can be downloaded from Google Play and F-Droid, among other sources.
Email features
Thunderbird can manage multiple email, newsgroup, and news feed accounts. It also allows users to use different identities within the same account. Features like quick search, saved search folders ("virtual folders"), advanced message filtering, message grouping, and tags help users organize and find messages. On Linux-based systems, system mail (movemail) accounts were supported until version 91.0. Thunderbird offers basic support for system-specific new email notifications and can be expanded with more advanced notification options using an add-on.
Thunderbird uses a Bayesian spam filter, a whitelist based on the included address book, and can understand classifications from server-based filters such as SpamAssassin.
Thunderbird follows industry standards for email. Email providers, such as Gmail and Outlook, have increasingly used OAuth authentication instead of traditional methods. Thunderbird fully supports OAuth authentication and provides instructions for Outlook users switching from the now-outdated "basic authentication" method.
Thunderbird can support different mailbox formats using plugins. However, this feature is not currently active because the necessary work is still being done. As of July 2014, the supported mailbox formats included:
Thunderbird also uses Mork and (since version 3) MozStorage (based on SQLite) for its internal database. Mork was planned to be replaced with MozStorage in Thunderbird 3.0, but the 8.0 release still uses the Mork file format.
Since version 13, Thunderbird has included support for uploading files to online storage services. Users can link large files instead of attaching them directly to an email. Supported providers included Ubuntu One and Dropbox. Currently, file linking can be done through add-ons or by connecting to a WebDAV provider.
Thunderbird includes a wysiwyg editor for creating messages formatted with HTML (the default). It can automatically send unformatted messages as plain text, based on user preferences. Special formatting like subscript, superscript, and strikethrough is available from the Format menu. The Insert > HTML menu allows users to edit the HTML source code of a message. Thunderbird also has basic support for HTML template messages, which are stored in a dedicated templates folder for each account.
Markdown support is available through the Markdown Here Revival add-on.
Thunderbird provides security features such as secure TLS/SSL connections to IMAP and SMTP servers. It also supports secure email through digital signing and message encryption using OpenPGP (with public and private keys) or S/MIME (with certificates). These security features can work with smartcards if additional extensions are installed.
Other security features can be added through extensions. Before version 68, the Enigmail extension was needed for OpenPGP support (now built-in).
Optional security protections include disabling remote images in messages, limiting media types (sanitizer), and disabling JavaScript.
The French military uses Thunderbird and helps improve its security features. These features are said to meet the standards required for NATO's secure messaging system.
Like all software, Thunderbird may have limits on the number and size of files and objects it can handle. For example, POP3 folders may be affected by filesystem design limits, such as maximum file sizes on systems without large-file support, or issues with long filenames.
Other features
Thunderbird is mainly used as an email program. It also has a feature called News Feeds and Blogs, which helps collect news. This feature works with RSS and Atom formats.
Thunderbird includes a chat tool for instant messaging. It supports IRC, XMPP, Matrix (added in version 102), and Odnoklassniki. Earlier versions also worked with Twitter, Google Talk, and Facebook Chat.
Thunderbird can also read Usenet news using NNTP and NNTPS. It is designed for reading, not for downloading files using NZB. It also does not have a search tool for newsgroups.
Starting with version 91, Thunderbird added a built-in PDF viewer. This allows users to open and read PDF files directly in the program without needing another program.
Cross-platform support
Thunderbird is available on many different types of computer systems. Official versions can be downloaded from the main website and work with the following operating systems:
Unofficial versions are also available for:
Older versions of Thunderbird can be used on OS/2, including ArcaOS and eComStation.
The code that makes Thunderbird work is freely available. This code can be used to create versions of Thunderbird that run on different types of computer systems and operating systems.
In June 2022, the Thunderbird project announced a partnership with K-9, an email app for Android phones, to develop a version of Thunderbird for Android. Christian Ketterer, the lead developer of K-9, joined the Thunderbird team in 2022. The plan was to change K-9 Mail into a version of Thunderbird for Android, including using the Thunderbird name and branding. According to Jason Evangelho of Thunderbird, this change would happen once K-9 Mail matched Thunderbird's features and design.
In the 2022 Financial Report published on May 8, 2023, Ryan Sipes, the Product and Business Development Manager, said the team planned to hire an iOS developer later in 2022 to begin creating a version of Thunderbird for iOS.
On October 30, 2024, the first stable version of Thunderbird for Android, called Thunderbird Mobile, was released as version 8.0. This version can be downloaded from Google Play and F-Droid, among other sources. The Thunderbird team planned to support K-9 for one year after the release of Thunderbird Mobile, which would be until around October 2025.
On May 6, 2025, the stable version 10 of Thunderbird Mobile and the beta version 11 were released.
As of 2025, an early version of Thunderbird's iOS app was available through Apple's TestFlight.
History and development
Thunderbird was first called Minotaur when it started, right after Phoenix, which was the original name for Mozilla Firefox. However, the Minotaur project did not become popular. With the success of Firefox, people wanted a mail client to use with it, so work on Minotaur was restarted under the new name Thunderbird. It was also moved to a new set of tools developed by the Firefox team.
On December 7, 2004, version 1.0 of Thunderbird was released. It had more than 500,000 downloads in the first three days and over 1,000,000 downloads in ten days.
Significant work on Thunderbird began again when it was announced that, starting with version 1.5, the main Mozilla suite would be made up of separate applications using the new tools. This was different from the old way of combining all tools into one program. The original Mozilla Suite continued to be developed as SeaMonkey.
On December 23, 2004, Project Lightning was announced. It added calendar features like scheduling and tasks to Thunderbird. Lightning supports the same calendar systems as the Mozilla Calendar infrastructure, just like modern versions of Sunbird.
On October 11, 2006, Qualcomm and the Mozilla Foundation announced that future versions of Eudora would use the same technology as Thunderbird. This project was called Penelope.
In late 2006, Debian changed the name of Thunderbird to Icedove because of trademark and copyright issues. This was the second product to be renamed.
On July 26, 2007, the Mozilla Foundation said Thunderbird would be developed by an independent group. This was because the Mozilla Corporation, a part of the foundation, was focusing on Firefox.
On September 17, 2007, the Mozilla Foundation announced funding for a new internet communications project with David Ascher of ActiveState. The goal was to create software based on Thunderbird’s code and brand.
On February 19, 2008, Mozilla Messaging started as a part of the Mozilla Foundation. It was responsible for developing email and similar tools. Its first focus was on the upcoming version of Thunderbird 3, which had early releases called "Shredder."
On April 4, 2011, Mozilla Messaging was combined with the Mozilla Labs group.
In 2011, Thunderbird changed to a faster release schedule, like Firefox. Version 4.0 was skipped to match Firefox’s version 5.0, both released in June 2011. By the end of the year, Thunderbird reached version 9.0.
On July 6, 2012, a secret message from Jb Piacentino, Thunderbird’s managing director, was leaked to TechCrunch. It said Mozilla would reduce its team working on Thunderbird and let the community handle new features. Later, Mozilla announced it would stop prioritizing Thunderbird development because adding new features was not effective. Instead, Mozilla would only provide security updates, while the community would take over new features.
On November 25, 2014, Kent James of the Thunderbird Council said that volunteers and developers met in Toronto to plan Thunderbird’s future. They decided more full-time staff were needed to make Thunderbird stable and improve features requested by users.
On December 1, 2015, Mitchell Baker, Mozilla’s executive chair, said Thunderbird development should be separate from Firefox. She explained that Thunderbird developers spent too much time supporting changes to Mozilla tools, while Firefox was helping Thunderbird. She also said Thunderbird did not have the same potential as Firefox. Mozilla still wanted to support Thunderbird but needed more help. At the same time, Mozilla said it would provide temporary legal and financial support for the project.
On May 9, 2017, Philipp Kewisch said the Mozilla Foundation would continue to support Thunderbird legally and financially. However, Thunderbird would no longer use Mozilla Corporation’s systems. Mozilla brought Thunderbird back into its own team, added new members, and improved security and the user interface.
In late 2017, Thunderbird 57 and 58 started using changes from Firefox Quantum, including a new "Photon" interface. However, Thunderbird 60 in 2018 still supported old add-ons, even though most needed updates. WebExtensions were not fully supported except for themes. In 2018, work began to support WebExtensions in Thunderbird 63 while still allowing some old add-ons.
In 2019, Thunderbird 68 only supported WebExtensions. Users could still use old add-ons by enabling a special "legacy mode," but these add-ons needed to be updated. OpenPGP support was added directly to Thunderbird, replacing the Enigmail extension. This was done using the RNP library instead of GnuPG because of licensing rules.
On January 28, 2020, the Mozilla Foundation said Thunderbird would operate through a new company, MZLA Technologies Corporation. This allowed Mozilla to explore new products and earn money through partnerships and donations.
As of version 78.7.1, Thunderbird no longer supports Legacy WebExtensions. Only MailExtensions, which are WebExtensions with Thunderbird-specific features, are compatible.
Thunderbird 91 includes improved user interfaces, support for Apple silicon, CardDAV address books, built-in tools for importing and exporting profiles, the PDF.js PDF viewer, and the ability to encrypt emails to BCC recipients.
On June 13, 2022, the Thunderbird team announced it would take over development of K-9 Mail, an Android email app. The plan was to turn it into a mobile version of Thunderbird with synchronization support.
In November 2024, Thunderbird Mobile became available for download. In December 2024, iodéOS, an open-source Android operating system, included Thunderbird Mobile as the default email client for over 30 devices.
Thunderbird releases happen in three stages: Beta, Earlybird, and Daily. These match Firefox’s Beta, Aurora, and Nightly stages.
Thunderbird Pro
On April 4, 2025, the Thunderbird team shared plans for a new set of paid, cloud-based services called "Thunderbird Pro." The team described these services as an open-source system focused on protecting user privacy, designed to compete with platforms like Google Workspace and Microsoft 365. They explained that the main Thunderbird email program will stay free, while Thunderbird Pro will be an optional, separate service offering features not possible in a basic email client, such as server-side processing and storage.
The plan for Thunderbird Pro is a subscription-based model to pay for operating costs. The team mentioned they may add a limited free version once the service becomes stable. They stressed that all parts of the service will be open source and built using open standards to prevent users from becoming dependent on a single company.
The services included in the Thunderbird Pro suite are:
At the time of the announcement, the Appointment service was in a closed beta test, Send was in an early testing phase (alpha), and a waitlist had opened for the Thundermail beta.