FreshRSS 
Welcome back from the Thunderbird development team!
The past few months have been exceptionally busy across the project. As we approach the midpoint of the year, we’ve been focused on a mixture of delivering user-facing features, investing in long-term architectural improvements, and preparing for the next ESR cycle.
A significant amount of effort has gone into modernizing Exchange support, where the team is now approaching Graph API feature parity with our existing EWS implementation. At the same time, progress has continued on the Account Hub, the Global Message Database, and improvements to the add-ons ecosystem that will help extension developers transition toward a more secure and sustainable future.
Behind the scenes, we’ve also continued the less visible but equally important work of maintaining a large application: adapting to upstream platform changes, improving test reliability, addressing long-standing bugs, and supporting the growing community of contributors who help move Thunderbird forward every day.
This month we’d especially like to recognize one of those contributors, Maxe, whose sustained efforts tackling decades-old MIME bugs have been making a meaningful impact across the codebase.
Exchange Email Support
One of the largest efforts underway in Thunderbird continues to be our modernization of Exchange support.
Over the past several months, the team has pushed through multiple Graph API implementation phases and is now entering the final stretch toward feature parity with our existing EWS implementation. At the time of writing, only a small number of remaining email features separate the two implementations, with completion expected imminently.
Reaching this point has involved considerably more than simply implementing new API calls. The work required substantial investment in shared understanding, protocol abstractions, automated code generation, testing frameworks, request batching, synchronization mechanisms, and interoperability between legacy and modern components. Many of these improvements will continue to benefit future protocol work long after Graph support itself is complete.
A notable development came from our ongoing engagement with Microsoft, and following discussions around Graph API permissions, Microsoft confirmed that approved mail clients such as Thunderbird will continue to be able to obtain user consent for permissions that were previously unavailable to third-party applications. This removed a significant long-term uncertainty around Graph support and helps to ensure Thunderbird users can continue connecting Exchange accounts without requiring administrator intervention.
With email functionality nearing completion, the team has already begun planning the next stage of Exchange support, including calendar integration work that will build upon the foundation established over the past year.
Keep track of our Graph API implementation here.
20+ year old MIME bugs?! – Contributor Spotlight
This month we’d like to highlight Maxe, who has been on an impressive run tackling some of Thunderbird’s oldest and most stubborn MIME issues.
Open source projects often benefit from contributors who quietly and consistently improve areas of the codebase that most people would rather avoid. Over the past several months, Maxe has become one of those contributors for Thunderbird.
What began as a handful of fixes has grown into a sustained effort to tackle some of the oldest MIME-related bugs in our tracker. Many of these issues date back decades, touching parts of the mail stack that have accumulated years of edge cases, historical assumptions, and compatibility quirks.
MIME handling sits at the heart of how Thunderbird interprets messages, attachments, encodings, and content types. While users rarely think about it when everything works correctly, it is often involved when messages display incorrectly, attachments behave unexpectedly, or unusual emails expose long-standing inconsistencies. Fixing these issues requires a deep understanding of both email standards and Thunderbird’s historical behavior.
What has impressed us most is not any single patch, but the consistency. Over the past few months Maxe has continued to identify issues, develop fixes, respond to review feedback, and refine solutions until they work reliably across platforms and message types. Along the way, several fixes have uncovered additional problems and improved behaviour in places that weren’t originally expected.
This kind of work is rarely flashy. It involves patiently navigating decades-old code, reproducing obscure bugs, and developing enough confidence to modify systems that affect virtually every Thunderbird user. Yet these are exactly the sorts of contributions that make open source software better over the long term.
On behalf of the team, thank you Maxe for the energy, persistence, and technical skill you’ve brought to Thunderbird this year. Your work is making a real difference.
Add-ons, Extensions and Ecosystem
The add-ons ecosystem remains an important part of Thunderbird, and over the last few months we’ve continued working toward a safer and more maintainable extension platform.
One significant decision was the postponement of experiment deprecation on the Monthly Release channel for an additional year. Feedback from extension developers made it clear that many maintainers needed more time to migrate away from legacy experiment APIs, and we want to ensure that transition is successful rather than disruptive.
This extra time allows us to focus on expanding official WebExtension APIs, improve migration paths, and work directly with extension developers to understand their priorities. To support this effort, we’re preparing a broader outreach initiative later this year that will gather feedback from experiment maintainers and help guide future API development.
A great deal of this work has been driven by John, who has been balancing ecosystem improvements alongside onboarding new team members and supporting several other strategic projects. Ensuring that extension developers have a sustainable path forward remains a key investment area for Thunderbird.
Authentication and OAuth
Over the past several months we’ve continued modernizing Thunderbird’s authentication experience, with a particular focus on OAuth and account setup.
One of the most visible improvements has been the continued rollout of browser-based OAuth flows. Instead of embedding authentication within Thunderbird itself, users can now complete sign-in using their system browser, providing a more familiar experience while benefiting from the security features and account state already present in their preferred browser.
As we expanded support for these flows, we also uncovered an interesting interoperability challenge. RFC 8252, the standard commonly used by native applications, recommends the use of loopback redirects with dynamically assigned local ports. While most providers support this approach correctly, several major providers have historically handled these redirects differently. As a result, we’ve been working directly with providers including Yahoo!/AOL, Comcast/Xfinity, and Yandex/Mail.ru to improve compatibility and ensure Thunderbird users continue to enjoy a smooth sign-in experience as authentication requirements evolve.
We’ve also been simplifying account setup for users of Thunderbird’s growing ecosystem of services. Recent work allows users to launch authentication for a Thundermail account directly from Thunderbird without first manually entering account details. This significantly streamlines onboarding and lays the groundwork for similar experiences with other major providers in the future.
Another important addition has been the introduction of a Thunderbird-specific protocol handler. This enables web-based account dashboards, management interfaces, and enterprise deployment tools to communicate directly with Thunderbird and complete account configuration automatically. For Thundermail users, this creates a much smoother path from account creation to a fully configured desktop client. Looking ahead, the same technology opens the door to deeper integration opportunities for enterprise deployments and other hosted services.
While much of this work happens behind the scenes, it represents an important investment in making account setup faster, more reliable, and more secure for both individual users and organizations deploying Thunderbird at scale.
Panorama – Global Message Database
Behind the scenes, work continues on one of Thunderbird’s most ambitious long-term architectural projects: the Global Message Database.
Recent months have focused on strengthening the foundations needed to connect Panorama’s user experience with the underlying storage architecture. Geoff has resumed significant front-end work following ESR-related priorities, while Brendan has joined the project to help accelerate development and planning efforts. At the same time, Ben has been refactoring portions of the IMAP codebase to establish cleaner interfaces that will simplify integration with the new database architecture.
While much of this work remains infrastructural and therefore less visible to users today, it represents important progress toward a more modern foundation capable of supporting future performance, search, and organizational improvements throughout Thunderbird.
Maintenance, Upstream adaptations, Recent Features and Fixes
While major features tend to attract the most attention, a significant portion of Thunderbird’s engineering effort continues to be devoted to maintenance and adaptation work required to keep pace with our upstream platform.
This period is traditionally one of the busiest times of the ESR cycle. As Firefox prepares its next ESR release, large volumes of platform changes land in a relatively short period of time. While these improvements benefit Thunderbird in the long term, they can also introduce unexpected regressions, styling inconsistencies, test failures, and compatibility issues that require immediate attention.
One particularly notable example has been Mozilla’s ongoing Nova initiative, which introduces substantial visual and styling changes throughout Firefox. Without intervention, many of these changes would create inconsistencies across Thunderbird’s user experience. Richard (Paenglab) has done exceptional work identifying, triaging, and adapting these upstream changes to ensure Thunderbird continues to present a coherent and polished interface. Much of this work goes unnoticed when done well, which is perhaps the highest compliment for maintenance engineering.
Alongside these adaptation efforts, the team and contributor community have continued landing a steady stream of reliability, stability, and usability improvements across the application. Recent highlights include:
- Improvements to threaded message handling and sorting behaviour.
- Fixes for long-standing IMAP synchronization and data integrity issues.
- Improvements to POP3 reliability, including protections against queue deadlocks.
- Multi-monitor and mixed-DPI fixes for mail notifications.
- Continued migration work to Fluent as part of our localization modernization efforts.
- and many more which are listed in release notes for beta.
If you would like to see new features as they land, and help us find some early bugs, you can try running daily and check the pushlog to see what has recently landed. This assistance is immensely helpful for catching problems early.
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Toby Pilling
Senior Manager, Desktop Engineering
The post Thunderbird Monthly Development Digest: June 2026 appeared first on The Thunderbird Blog.
