• Development

Thunderbird Monthly Development Digest – June 2025

Hello once more from the Thunderbird development team! For many of our team members, the summer has started with our annual sprint to release ESR and enjoy a little time afk, as our colleagues in the southern hemisphere hunker down for winter and power through a pile of work down under.

Extended Support Release is alive!

For enterprise users (and those who have been using Thunderbird for a long time and found themselves on the Extended Support Release channel), the annual release “ESR 140 Eclipse” has made it to our update servers and will be pushed out over the next few weeks. 

We had initially planned to release within hours or days of the Firefox ESR release, but much of the ESR build process has changed in the last 12 months (largely due to the Firefox mozilla-central Git migration) – so we ended up learning a lot and took pause to release at a time that produced the best experience for the majority of our users. 

In the hours following the initial release, we have another build hot on its heels which includes some important patches and will ship today or tomorrow. Things move fast around here!

If waiting a year doesn’t sound appealing to you, our Monthly release may be better suited. It offers access to the latest features, improvements, and fixes as soon as they’re ready. Watch out for an in-app invitation to upgrade or install over ESR to retain your profile settings.

Exchange support in Daily

The EWS 0.2 milestone has been completed and the feature was turned on by default in Daily release to facilitate more manual QA testing. In order to provide test coverage on a variety of EWS server versions and configurations, we’re tackling in a few ways:

• Adding a small number of Hosted Exchange 2016 mailboxes to facilitate testing of all existing functionality at endpoints other than O365.
• Contacting enterprise partners who can help us test on their infrastructure – please get in touch if this might be you!
• Hosting our own EWS instance that allows us to configure a variety of security and authentication settings to ensure our code works for all
• Focusing on automated test coverage throughout the month of July

Since my last update, the team has grown even more and made great progress on items in our “Phase 2 operations” and “Phase 2 polish” milestones, with these features delivered recently:

• EWS-to-EWS move & copy for items and folders
• Authentication Error handling
• Server Version handling
• Threading support
• Folder updates & deletions during sync operations
• Folder cache cleanup
• Folder copy/move
• Bug fixes!

We plan to temporarily expand the team during July to include two more of our most experienced senior engineers to push this project over the finish line and tackle some remaining complexities:

• Automated test coverage
• Message filtering
• Setting as Junk/Not Junk
• Bug backlog

Keep track of feature delivery here.

Account Hub

The new email account feature was enabled as the default experience for users adding their second email account. It is now available in all release channels. We’re currently finalizing the UX and associated functionality that detects whether account autodiscovery requires a password, and reacts accordingly – which will hopefully be uplifted once stable.

We’re wrapping up the redesigned Account Hub UI for Address Book account additions this week, which we’ll enable for users on Daily and beta in the coming weeks. Look out for it in our Monthly release 142.

Global Message Database

Since the last update, we’ve landed a landslide of patches. Critical refactoring continues to clean and optimize the code, in many cases clearing the way for new beneficial protocol implementations.

To follow their progress, take a look at the meta bug dependency tree. The team also maintains documentation in Sourcedocs which are visible here.

Recent Features and Fixes

A number of other features and fixes have reached our Daily users this month. We want to give special thanks to the contributors who made the following possible…

• Even more improvements to notifications!
• Many, many port bugs to adapt Thunderbird to upstream changes
• Fix spell check and other windows impacted by Firefox code updates
• and many more 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

Thunderbird