Betterbird Blog

The article picture shows the effects of two recent regressions in Thunderbird 153.0.2 (ESR).

The left part shows what happens when trying to drag a message or message attachment to the desktop or a folder: Unspecified error. This is tracked in this ticket. The breakage was caused by the Mozilla team when fixing a security issue in Firefox 155; this fix was backported to Firefox 153.0.2 and also hit Thunderbird.

The right part shows what happens when dragging an image in a compose window to a different location or another compose window: The internal representation of the image is inserted as text. This is tracked in this ticket. The breakage was caused by the Mozilla team implementing changes to drag and drop for Firefox 151 in March 2026. Here QA has completely failed, since no one detected the breakage with affects both Firefox and Thunderbird since version 151 until the end of July 2026.

To understand what has happened, let's dive into how code for Firefox, Thunderbird and Betterbird is structured.

Firefox at version 153 consists of roughly 40 million lines of code. This includes code for the Firefox frontend (tab bar, address bar, bookmarks, and other Firefox-specific UI); Firefox mobile; the SpiderMonkey JavaScript engine; the DOM implementation; the layout and graphics pipeline covering CSS style resolution; the WebRender compositor, font rendering, and image decoding; the networking stack handling HTTP, DNS, and sockets; the NSS cryptography library providing TLS and certificate handling; the media engine for audio, video, and WebRTC; the XPCOM component model, which bridges JavaScript and C++ and provides cross-platform file I/O, threading, string handling, and process management; the widget layer that abstracts native windowing, event loops, clipboard, and drag-and-drop across Windows, macOS, and Linux; internationalisation; the shared toolkit providing preferences, add-on management, password storage, printing, downloads, and other cross-application services (Places, Find-in-page, Search engines, Autocomplete, Crash reporting, Telemetry (the latter two not being used in Betterbird)); and additionally the developer tools.

Of these 40 million lines, five million lines covering the Firefox frontend and Firefox mobile, are not shared by Thunderbird/Betterbird. Thus, these programs use 35 million lines of the so-called Mozilla platform code and two million lies of code of their own. Those two million lines cover the frontend and the backend with the following functionality: IMAP, POP3, Exchange and NNTP protocol implementations; the message database layer (including the legacy Mork format, the newer Gloda global-message-database and the future Panorama database); MIME parsing; message composition including HTML editing; the address book with LDAP and CardDAV; OpenPGP and S/MIME encryption; mail import/export; also included are Calendar (local calendars, CalDAV, invitations and scheduling, import/export) and Chat (IRC, XMPP, (defective) Matrix).

As mentioned above, the two regressions were caused by changes in the Mozilla platform code which is geared mostly towards Firefox. There was little chance to catch the first one since it was implemented for Firefox 155 at the end of July 2026, and was integrated into earlier branches, like the 153 release and ESR branches, almost immediately. Of course, an automated test would have caught it straight away, but despite touting their test suite, the Thunderbird test suite has many holes, also for very important functionality. There were more than four months to detect the second issue, but it went undetected until very recently.

Betterbird 153.0esr users are not affected by the first issues and we have published 153.0esr "build 7" with a fix for the second issue, well ahead of Thunderbird.

In general, at Betterbird, we patch both the Mozilla and the Thunderbird codebase, whereas the Thunderbird folks only manage their own code and rely on Mozilla staff for backporting fixes in the Mozilla platform code. This leads to delays in fixing bugs in the ESR version of up to one year.

When the Betterbird CEO was still in charge of Thunderbird releases until 2019, Thunderbird didn't rely completely on Mozilla staff for backports, it could independently fix bugs in the ESR version. These days, the Thunderbird folks either don't care or lost the ability.

Let's fill the summer hole/silly season with some content.

Two days ago we switched to Betterbird 153 for productive use. At first, everything looked good, but then we noticed a few regressions:

Our suggested userChrome.css had an issue, the "cute icons" didn't work any more: That got fixed and an update was published here.

Next we discovered an issue in ThunderHTMLedit. The Mozilla folks had tightened security by disallowing blob:system workers. That change snuck into Thunderbird/Betterbird beta 6. We adjusted the add-on accordingly and published version 3.6.6.

Finally we noticed that after pasting a screenshot into a composition, the caret ended up at the wrong location before the inserted image. Fortunately, someone had already reported the issue and Thunderbird's new star contributor flexed his AI muscle and provided a fix which we're currently evaluating.

Today we received this issue about Thunderbird's tags folder pane mode. This was half-implemented for Thunderbird's Supernova Kaboom version 115, and has been defective ever since the day of first shipping, as a few bugs ([1] [2] [3) show.

And while digging around Bugzilla, we found these pearls:

• Enable the defective tags mode by default: Abandoned by Thunderbird's own "Director, Desktop & Mobile Apps" - good one!
• Underline links: Looks like another Supernova victim. In Betterbird, setting the preference layout.css.always_underline_links will always underline links, also in message content.
• And as predicted in the article that states that Thunderbird development is driven by user complaints, those complaints about Thunderbird's poorly implemented account colours have now arrived. And the shonky design decision to not store the colours in preferences has come to bite.

What's happening at Betterbird during that the Germans call summer hole (Sommerloch), also called silly season in parts of the world?

The new ESR version 153 shipped about 10 days ago, and so far we had only little feedback, but all the feedback received was positive. Version 140.13.0-bb25 was also shipped around the same time.

We're constantly monitoring developments at Thunderbird, so a few days ago we shipped a build 2 for Betterbird 153.0esr-bb6. In that build, we shipped two minor corrections and additional upstream fixes.

While things are generally quieter, we've used the time to upgrade our macOS build machine to a Mac mini M4. This delivers faster builds, and keeps up with Apple's ever-changing Xcode requirements, and thus ensuring that Betterbird continues to build natively for Apple Silicon; it also allows a cross-compile for Intel Macs.

If you've been enjoying Betterbird, please tell a friend or colleague about it. And if you'd like to help keep the project going, consider sending a donation. Summer is traditionally a bit of a donation lull, so every contribution is especially appreciated. We don't run three or four yearly donation drives like the Thunderbird folks, only one in December.

We've shipped Betterbird 140.13.0esr-bb25 today. Please refer to the Release Notes for full details.

The Thunderbird had quite some trouble while building the release as mentioned in this earlier article. In fact, as they forgot to update the version number, their first successful build, not mentioning about eight failed builds, was built as 140.12.1/build2. Go figure. Three builds later they finally produced a correct build. However, the underlying Gecko version is already at 140.14.0, which could potentially affect some add-ons which are configured to only function to version 140.13.0.

Back to the Betterbird release. This release contains fixes some important bugs:

Zooming of images in links is back, after being removed from version 140.12.0, but this time controlled via preference mail.allow_zooming_image_links.

Spellcheck now works when the desired languages are stored in preference spellchecker.dictionary as a comma-separated list, for example: en-US,de,es-ES.

As mentioned, full details in the Release Notes.

It's been more than a month that we talked about regressions on the Thunderbird release channel in this article. So here is the next installment.

Careless refactoring in this bug caused a total of four regressions.

One of them, reported in bug 2040638, has clocked up 18 duplicates from affected users. The symptoms revolved around messages with attachments which caused failures when sending a or saving as draft. The second issue reported as bug 2050589 manifested itself in a stuck progress window, see article image.

Both bugs were fixed by Thunderbird's new star volunteer contributor while full time employees turned a blind eye or are enjoying their well-earned holidays. Needless to say that release channel users are still suffering as Thunderbird release management has not made the fixes available on this channel. Even on beta 153, only one of them was shipped. Both fixes are of course included in Betterbird's beta release.

We've lamented the squanderous allocation of Thunderbird resources before, so here is the next installment.

This mailing list post just hit the so-called Thunderbird UX mailing list. They author announces a redesign with the aim to simplify the Calendar UI, amongst other things. So what they have done is actually to remove functionality. But they author of the article preempts any concern by stating (quote):

These changes will be completely optional, kept in a separate “New Calendar” space, so those who need legacy features won’t lost access to any of their workflows.

How useful, so the rework will either be hidden, or users will hide it to get the functionality back that they need.

Thunderbird DOES NOT NEED yet another redesign and refactoring, what it needs is that the 2,538 open Calendar bugs be properly triaged and the most important ones fixed, like these:

Full list here, sort by Votes.

The Calendar component has been mostly abandoned since its Module Owner has moved onto greener pastures in so-called Thunderbird services (Thundermail, etc.). So now, instead of providing real functionality improvements, the Thunderbird folks have switched to polishing the turds.

Since the AI bubble is inexorably upon us, it's time to talk about it.

AI advances have boosted productivity in Thunderbird/Betterbird development. In the Thunderbird project, various folks have solved bugs with AI-assisted coding tools, powered by models such as OpenAI's GPT family, Anthropic's Claude models, Google's Gemini models, and DeepSeek models. They have analysed code, proposed and reviewed patches, written tests, and have even run the entire development pipeline via agents right up to running new tests and submitting work to Mozilla's Phabricator platform.

Our project hasn't spend big on AI subscriptions, but with humble subscription to DeepSeek v4 Pro, we've been able to analyse issues (see articles's screenshot), written scripts and automated tedious tasks like localisation, since one new string needs to be added to 15 localisation files and also be made consistent with existing translations in Thunderbird. So far, we haven't produced a Betterbird patch with AI yet, but we're looking forward to the first adequate task.

Update: We had DeepSeek revise our patch for showing native alerts on the application screen. AI pointed out some shortcomings wrt. to screen scaling and added code to cater for monitors aligned vertically. As a reminder, the function can be enabled with preference mail.notification.show_on_app_screen.

But AI is not only helping developers, users also benefit by having AI summarise messages or help when writing message by rephrasing, summarising, explaining, translating, etc. Here's a table of available add-ons.

Disclaimer: We've only surveyed listings on ATN and not tested those add-ons. There is no relationship between us and any of the add-on authors. If you are an add-on author and we misrepresented your add-on, please get in touch.

For e-mail processing, it makes sense to use local models, for example via Ollama. We also gave LM Studio a try, but found the model download unbearably slow on a fibre optic connection. Using a local model, e-mail data is not sent to the AI provider's data centre for processing.

Ollama users should note that in order for their add-on to be able to contact the local server at http://localhost:11434, they need to work around a CORS (Cross-Origin Resource Sharing) issue leading to a 403 error. Ollama needs to send the Access-Control-Allow-Origin: * header which can be enabled by setting the environment variable OLLAMA_ORIGINS to a value of *.

As widely announced on Thunderbird forums and mailing lists, Thunderbird is diversifying. First they bought the K-9 Android e-mail client and rebadged it as Thunderbird for Android. Next, they are developing an iOS mail application from scratch. The last undertaking is to offer so-called Pro Services, called "TB Pro": www.tb.pro. See the 2025 announcement here.

The mail hosting part of those services is called Thundermail. Currently, it's not yet available, but one can join a waiting list. Expect to pay 12 x USD 6 = USD 72 anually. Not a small price, given that for less money you can resister your own domain and pay a hosting service, or have your mail hosted at our advertising partner ForwardEmail. Of course, there are also long-standing e-mail services, like Fastmail, Posteo or Proton Mail, each backed up be years or decades of experience.

All that said, the Thunderbird folks are rolling out the integration of their Thundermail service in Thunderbird 153, currently in beta, see screenshot above. Technically, Thundermail is integrated as system add-on, which ships inside the application, packaged into omni.ja at chrome/messenger/builtin-addons/thundermail. It's enabled in the new Account Hub via preference mail.accounthub.thundermail.enabled.

The Thunderbird folks are shipping their version 153.0 beta 2 with Thundermail enabled. We're currently disabling Thundermail via the mentioned preference for various reasons:

• It doesn't yet appear to be available.
• We're not convinced that this is the best possible mail hosting offer.
• Our project does not aim to increase Thunderbird income, but to improve user experience.

Most of the source code of Thundermail is only published in minified/obfuscated form. We did a quick AI analysis of what is publicly visible, and here's the summary of the US services involved in the setup:

Any Betterbird user wishing to try or use Thundermail can easily set the preference to true.

Update 25th June 2026: We've just been made aware that apart from showing "Sign in with Thundermail" during account creation, Thundermail is also forced onto the application menu:

The DOM ID of this element is tbpro-menu-id-root; it is added here.

The add-on code is extracted from this repository and pushed into the Thunderbird tree. Here it is:

https://hg-edge.mozilla.org/comm-central/file/tip/mail/extensions/builtin-addons/thundermail/extension.

This code is not indexed in Searchfox.

Packaging software some or most users don't want into the application via some system add-on sounds like something Mozilla has done before? Well, check this Lunduke video.

Today we're starting our beta program for Betterbird 153 with 153.0b1-bb1. Please refer to the Release Notes for full details.

We're shipping this only in English and for Windows. Language packs for other languages are available.

Note that we're just starting to gain experience with the new 153 series, so please do not use this version in production yet. Also note that Thunderbird migrated password storage from logins.json to logins.db, so when downgrading, you need to re-enter all your passwords (or restore the previous logins.json file).

We've shipped Betterbird 140.12.0esr-bb24 today. Please refer to the Release Notes for full details.

This release contains fixes some important bugs:

Cancel sending had broken in Thunderbird 135, and no one noticed. We've already written about it. This is fixed in Betterbird 140 now, the Thunderbird folks fixed it for release 153.

Secondary sort is not a well-known feature. To make it a bit more obvious, Betterbird shows secondary sort indicators on column headings in table view where appropriate. Those weren't aligned correctly in default and relaxed densities.

Betterbird had a widely misunderstood feature where the user could zoom images in links if they were oversized. We had a few complaints since people didn't recognise the link, and if they did, had to used the context menu to open it. Finally a user sent us a sample e-mail which was rendered quite incorrectly due to the feature. That was the last nail in the coffin and the feature was dropped.

Lastly this release fixes a few issues with reversed sorting and collapsing a thread to the most recent message. This is driven be preferences mailnews.reversed_threading and mailnews.collapse_thread_to_most_recent. Check this article for details about those function.