HowItWorks:Thunderbird

Explains how things actually work — part of our technology dissections collection.

Thunderbird is a free and open-source, cross-platform personal information manager, regrouping email, calendar, contacts, RSS feeds, chat and newsgroups in one application.

What is Thunderbird?

Thunderbird is a desktop personal information manager. It manages email accounts over the POP3 and IMAP protocols, sends outgoing mail over SMTP, and additionally ships a calendar, an address book, an RSS/Atom feed reader, a newsgroup reader and chat support.

Three values guide the project, as stated on its about page:

• Privacy — "You are not the product." Thunderbird does not collect or sell user data, and users retain ownership of it.
• Freedom — Thunderbird is free and open-source software; its license, the Mozilla Public License 2.0, "ensures that it will remain free forever".
• Voice — anyone can get involved, and a contributor-elected council keeps Thunderbird "true to its values and mission".

Thunderbird runs on Windows, macOS and Linux.

A short history

From the first releases to 1.0 (2003–2004)

The Thunderbird project was founded in 2003. The release history on thunderbird.net starts at version 0.1 and runs through 0.9 before the 1.0 milestone.

On December 7, 2004, the Mozilla Foundation announced the worldwide availability of Thunderbird 1.0 for Windows, Mac OS X and Linux, "focused on new features and settings to help stop spam and prevent viruses, the two biggest problems facing email users today". The release introduced adaptive junk mail controls, an integrated RSS news and blog reader, saved search folders, themes and extensions, and easy migration from Outlook, Outlook Express, Eudora and Netscape Communicator. "We view Thunderbird as a big step towards providing people with a better email experience", said Scott MacGregor, the project's lead engineer.

Community stewardship (2012–2015)

On July 6, 2012, Mozilla chairman Mitchell Baker announced a new direction: Mozilla's leadership had concluded that "on-going stability is the most important thing" for Thunderbird and that "continued innovation in Thunderbird is not a priority for Mozilla's product efforts". Mozilla would provide security updates through an Extended Support Release process, and the community could organize for ongoing development.

The Thunderbird project later called the decision "both a blessing and curse": the community kept the project alive and contributed many features, but a volunteer-driven effort without a proper organization accumulated an inconsistent interface and mounting technical debt.

In 2014, contributors established the Thunderbird Council, an elected body "functioning much like a board of directors for an open-source project": it oversees funds, approves budgets and shapes the product roadmap. In June 2015, Thunderbird 38 shipped calendaring by default, implemented as the Lightning extension.

Finding a home (2015–2020)

In late 2015, Mitchell Baker opened a discussion about Thunderbird's future. Mozilla would "disentangle the technical infrastructure" and assist the Thunderbird community, with the Mozilla Foundation serving as fiscal sponsor for donations. Mozilla commissioned a report from Simon Phipps, former president of the OSI, on potential new homes; on May 9, 2017 the Thunderbird Council announced the conclusion: the Mozilla Foundation would remain the project's legal and fiscal home, but Thunderbird would migrate off Mozilla Corporation infrastructure and operate independently, serving "over 25 million users".

On January 28, 2020, the project moved again: Thunderbird began operating from MZLA Technologies Corporation, a new wholly owned subsidiary of the Mozilla Foundation, to "hire more easily, act more swiftly, and pursue ideas that were previously not possible", including partnerships and paid services, while remaining free and open source.

Rebuilding: Supernova, Android, monthly releases (2023–2026)

In February 2023, the project announced a ground-up rework with three objectives: make the code base leaner and remove technical debt, rebuild the interface from scratch, and switch to a monthly release schedule. On July 11, 2023, Thunderbird 115 "Supernova" shipped: "a modernized overhaul of the software, both visually and technically". "Supernova is the beginning of a new era for Thunderbird", said Ryan Sipes, Thunderbird's Product and Business Development Manager. Supernova introduced a card view for the message list, a dynamic unified toolbar, a redesigned application menu, sortable folder modes, a tags view, and substantially improved keyboard and screen-reader accessibility.

On October 30, 2024, Thunderbird for Android 8.0 was released: the first stable release of the mobile app, developed from K-9 Mail, which the project took under its wing in 2022.

Thunderbird now follows a release train: a new major version every 4 weeks, a monthly release, and one Extended Support Release per year, typically in July. As of September 2026, the latest release is 154.0, with 153.0 and 140.0 as the supported ESR branches. Thunderbird for iOS is in development, described in the July 2026 progress report as the project's "first built from scratch" app.

Components and interactions

Thunderbird is built on the Mozilla platform. As the team describes it: "Thunderbird is literally a bunch of code running on top of Firefox. All the tabs and sections you see in our applications are just browser tabs with a custom user interface." This base supplies cross-platform support, the Gecko web renderer, the SpiderMonkey JavaScript engine, and inherits security patches and extension support from the Firefox release cycle; the cost is tracking a fast-moving upstream codebase with a far smaller team. The application itself is written in C++, JavaScript, CSS and XHTML.

Thunderbird's source code lives in the comm-central repository. Its main directories:

• mail/ — the Thunderbird-specific frontend: user interface files, shared JavaScript modules, and components such as the account setup wizard, the address book front end, the composition window, the chat front end and the preferences tab.
• mailnews/ — "source code specific to the Mail and Newsgroups part of Thunderbird and SeaMonkey": the protocol and storage backend.
• calendar/ — the calendar component, formerly known as the Lightning extension.
• chat/ — the chat component, with one subdirectory per protocol; related code also lives in mail/components/im.
• build/, testing/, taskcluster/ and third_party/ — build, test and vendored-code infrastructure.

Add-ons are MailExtensions built on the WebExtension technology used by web browsers: they are "created using standard JavaScript, CSS and HTML", use stable APIs that do not need updating on every release, and request permissions the user must confirm. Advanced add-ons can bundle "Experiment" APIs that interact directly with Thunderbird's internal APIs. Since Thunderbird 78 (July 2020), MailExtensions are the only supported add-on type.

Thunderbird in action

Account setup

Creating an account is a wizard flow. The user enters their name, email address and password; Thunderbird queries the ISPDB (Internet Service Provider Database), a Thunderbird-run service "used for auto configuring users' email accounts", to auto-fill the incoming and outgoing server settings.

The mail round trip

Incoming mail: Thunderbird connects to the server over IMAP or POP3, downloads new messages into the profile, updates the message database, and notifies the interface. Outgoing mail goes out over SMTP.

Key design decisions

Riding the Firefox platform

The founding architectural decision: build the mail client on top of the Mozilla platform rather than from scratch. Thunderbird inherits cross-platform support, the Gecko renderer, SpiderMonkey, networking and security patches from Firefox. The trade-off is a constant chase: with a bit more than a dozen core developers against Firefox's hundreds, "how many times per week things suddenly break in Thunderbird because a C++ interface was renamed, or an API was deprecated, or a building library was upgraded". This upstream churn, plus years of volunteer-driven additions, produced the technical debt that motivated the Supernova rebuild.

From XUL to web technologies

Thunderbird's interface was historically built with Mozilla-specific technologies: XUL, XBL and XPCOM. As early as 2017 the project's long-term plan was to migrate to "web technologies", meaning HTML and pure JavaScript, while remaining an installable desktop application. The Supernova release (2023) was the visible first step of that rework: "we reworked some of the oldest parts of our code base with the goal of modernizing them to make maintenance and extensibility easier".

Community governance, corporate backing

Thunderbird pairs an unusual governance structure with an equally unusual corporate one. The Thunderbird Council, elected by contributors since 2014, steers strategy and budget; development and operations are run by MZLA Technologies Corporation, a for-profit subsidiary of the not-for-profit Mozilla Foundation, which gives the project "the flexibility to offer optional paid services to sustain Thunderbird's development far into the future". After the uncertainty of the 2012 community handover and the 2015–2017 search for a home, this structure gave the project paid staff, a roadmap and a release cadence.

Stable, permissioned add-on APIs

Thunderbird's extensibility has long been a selling point, but legacy XUL/XPCOM add-ons broke with nearly every release. The project's answer was to adopt WebExtensions and call them MailExtensions: stable APIs, permission prompts, no forced per-release updates. Since Thunderbird 78, only MailExtensions are supported, and "restartless add-ons and non-restartless legacy add-ons using XUL overlays are no longer supported".

A predictable release train

Thunderbird follows a 4-week release cycle: a new Daily (nightly) major version begins every 4 weeks, the code merges into Beta, then into the monthly Release, with point releases as needed 2 weeks after. The ESR branch is refreshed once a year, typically in July, and receives point releases as needed every 2 weeks. This replaced the earlier yearly-major model which, combined with upstream Firefox changes, at times left Thunderbird unable to build or release "for months at a time".

Open standards and privacy

Thunderbird talks to the outside world through open standards: IMAP, POP3, SMTP, NNTP and RSS for content, CalDAV and iCalendar for the calendar, and open chat protocols. The project's privacy stance is uncompromising: "You are not the product", it does not collect or store user data unless asked, and it will never sell it.

Try it yourself

The current version can be checked against Mozilla's official product-details API (verified live on 2026-09-01):

# What is the latest stable release right now?
curl -s https://product-details.mozilla.org/1.0/thunderbird_versions.json

Output (verified live on 2026-09-01):

{
    "LATEST_THUNDERBIRD_ALPHA_VERSION": "54.0a2",
    "LATEST_THUNDERBIRD_DEVEL_VERSION": "156.0b1",
    "LATEST_THUNDERBIRD_NIGHTLY_VERSION": "157.0a1",
    "LATEST_THUNDERBIRD_VERSION": "154.0",
    "THUNDERBIRD_ESR": "140.14.1esr",
    "THUNDERBIRD_ESR_NEXT": "153.1.1esr"
}

Install the official Flatpak, which is "packaged and maintained by the Thunderbird team":

# Install from Flathub
flatpak install flathub org.mozilla.Thunderbird

# Launch it
flatpak run org.mozilla.Thunderbird

Or use your distribution's package:

# Debian / Ubuntu
sudo apt install thunderbird

All of Thunderbird's data lives in a single profile directory (on Linux, ~/.thunderbird): preferences, mail folders, address books and the message database. Migrating to a new machine is "really just a matter of copying the profile folder to the right place on the new machine".

Further reading

• Thunderbird — the official website
• The Thunderbird Blog — release announcements and project news
• Thunderbird Developer Documentation — architecture, building, add-ons
• comm-central — the source repository
• Thunderbird release notes
• Thunderbird support (SUMO)
• Thunderbird roadmaps
• Get involved