• Development

Thunderbird Monthly Development Digest: August 2025

Hello again from the Thunderbird development team! As autumn settles in, we’re balancing the steady pace of ongoing projects with some forward-looking planning for 2026. Alongside coding and testing, some of our recent attention has gone into budgets, roadmaps, and setting priorities for the year ahead. It’s not the most glamorous work, but it’s essential for keeping our momentum strong and ensuring that the big features we’re building today continue to deliver value well into the future. In the meantime, plenty of exciting progress has landed across the application, and here are some of the highlights.

Exchange support for email is here

Exchange support has officially landed in Thunderbird 144, which will roll out as our October monthly release. A big final push from the team saw a number of important features make it in before the merge:

• Undo/Redo operations for move/copy/delete
• Notifications
• Basic Search
• Folder Repair
• Remote message content display & blocking
• Status Bar feedback messaging
• Account Settings screen changes
• Autosync manager for message downloads
• Attachment delete & detach
• First set of advanced server settings
• Experimental tenant-specific configuration options (behind a preference) now being tested with early adopters

The QA team is continuing to work through their test plans with support from a small beta test group, and their findings will guide the documentation and support we share more broadly with users on monthly release 144, as well as the priorities to tackle before we head into the next chapter.

Looking ahead, the team is already focused on:

• Expanding advanced server settings for more complex environments
• Improving search functionality
• Folder Quotas & Subscriptions
• Refining the user experience as more real-world feedback comes in
• A planning session to scope work to support calendar and address book via EWS

Keep track of feature delivery here.

Conversation View Work Week

One of the biggest milestones this month was our dedicated Conversation View Work Week which recently wrapped up, where designers and engineers gathered in person to tackle one of Thunderbird’s most anticipated UX features. 

The team aligned early on goals and scope, rapidly iterated on wireframes and high-fidelity mockups, and built out initial front-end components powered by the new Panorama database. 

By the end of the week, we had working prototypes that collapsed threads into a Gmail-style conversation view, demonstrated the new LiveView architecture, and produced detailed design documentation. It was an intense but rewarding sprint that laid the foundation for a more modern and intuitive Thunderbird experience.

Account Hub

We’ve now added the ability to manually edit an EWS configuration, as well as allow for users to create an advanced EWS configuration through the manual configuration step

The ability to cancel any loading operation in account hub for email has been completed and will be added to daily shortly

• This also had the side effect of users who click “Stop” in the account old setup with an OAuth window open now closing the OAuth window automatically
• We will be uplifting this change to beta and then ESR

Progress is being made with adding a step for 3rd party hosting credentials confirmation, with the UI complete and the logic being worked on

• This progress will have to take into account changes from the cancel loading patch, as there are conflicting changes
• Once this feature is complete, it will be uplifted to beta, and then ESR

Work will soon be starting to enable the creation of address books through account hub by default.

Follow progress in the Meta Bug

Calendar UI Rebuild

After a long pause, work on the Calendar re-write has resumed! We’ve picked things back up by continuing focus on the event read dialog. A number of improvements have already landed, including proper handling of description data and several small bug fixes.

We have seven patches under review that cover key areas such as:

• Accessibility improvements, including proper announcements of event and calendar titles.
• Adding the footer for acceptance.
• Updating displays and transitioning current work to use the mod-src protocol.
• Handling resizing

Development is also underway to add attendee support, after which we’ll move on to polishing the remaining pieces of the read dialog UI.

Maintenance, Recent Features and Fixes

August was set aside as a focus for maintenance, with a good number of our team dedicated to handling upstream liabilities such as our continued l10n migration to Fluent and module loading changes. In addition to these items, we’ve had help from the development community to deliver a variety of improvements over the past month:

• Tree restyling following upstream changes – solved
• An 18 year old bug to enable event duplication via drag & drop – solved
• A 15 year old bug to sort by unread in threads correctly – solved
• Implementation of standard colours throughout the application. [meta bug]
• Modernization of module inclusion. [meta bug]
• 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 squash 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