Thunderbird ESR is here: Mozilla's email client adds new functions

The latest version of the messaging client from Mozilla subsidiary MZLA has a bunch of useful new features, and will get updated until mid 2026.

Thunderbird 140 "Eclipse" is not just an ordinary new version: it's the new Extended Support Release, which means that for the most cautious users this will be current until the middle of next year, and its packing some fresh functions.

The release follows May's Thunderbird 139, but it is not the same. As we covered when Firefox and Thunderbird 138 came out, Thunderbird joined Firefox in its approximately monthly release schedule back in February.

So now, for Thunderbird just like its sibling browser, there are two different and separate release channels: the slower-moving ESR channel, in which versions last for about a year and get just security fixes rather than new features, and the normal Release channel, which MZLA recommends. New releases arrived about every four weeks, and they also get point releases between times if there are urgent bugs or security issues.

Thunderbird 140 is both: it's got some significant new features, but it will also get updated for the next year, until Thunderbird 152 or thereabouts. The announcement calls out five features for special attention.

Dark Message Mode applies the app's existing dark mode to message bodies as well, overriding emails' own formatting. This vulture rarely uses dark mode, but if you do presumably this sounds good. This version consolidates appearance preferences from several different places into the Appearance tab of the Settings screen, including making it easier to switch from the new Card view that appeared in Thunderbird 115 to the traditional Table view. In Table view, it now supports horizontal scrolling.

The app now uses the host OS's native notifications on all three supported platforms, Windows, macOS and Linux, and allows you to act on messages direct from the new-message notification, such as archiving it or marking as read. Accounts management has been simplified and consolidated: adding any new account uses the new Account Hub wizard. Handily, folders can now be dragged to rearrange them without going into accounts settings.

One new feature that the Reg FOSS desk was really hoping to see has not arrived: we're still waiting for Thunderbird Sync, which we mentioned in 2023 and seems no closer, despite the occasional mention on the project blog.

This release's announcement also mentions a new feature which has been there for a while: desktop Thunderbird can show a QR code which you can scan using Thunderbird for Android. This immediately recreates all the accounts from the desktop version. We tested this when we looked at Murena /e/ OS 3 and it works very well. It's not as versatile as the equivalent to Firefox Sync we had hoped for, but it's a good big stride in the right direction.

A feature that is flagged as being still experimental is direct Microsoft Exchange support. This has been in development for quite some time. Thunderbird can already talk to Exchange servers: the problem is, it depends on how each specific Exchange box is configured. If Exchange's optional IMAP4 support is installed and enabled, Thunderbird can use that on its own. However, Microsoft's own clients don't use standard protocols, so many sites don't have that.

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If you don't mind paying for an inexpensive Thunderbird extension, another option is Beonex's Owl addon, which adds Exchange Server support. If the Exchange server is configured to allow browser access – sometimes called Exchange Web Services or Outlook Web Access (EWS or OWA) – then another Beonex tool, ExQuilla can use that to get Thunderbird talking to it.

ExQuilla's days are numbered, because Thunderbird can now natively talk to EWS, meaning it can access Exchange Servers without IMAP support and without addons. This spells the end for Beonex's add-on, but that clock was ticking anyway: Microsoft plans to turn off EWS support in October 2026 for the current versions of Microsoft 365 and Exchange Online. It is being turned off, along with other Microsoft web APIs, to be replaced with Microsoft's next-gen Graph API, which by then will be a full decade old.

The 2026 ESR version of Thunderbird is expected to arrive months before this Exchange feature goes away, so it has time to prove itself – and in any event, many sites will keep their older versions of Exchange around. ®

Bootnote

For those who are not so keen on the UI changes happening since Firefox "Supernova" (115), "Nebula" (128) and now "Eclipse" (140), then a new ESR release of Thunderbird means that a new release of Betterbird is coming. The beta of Betterbird 140 is already available.