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.