Thunderbird, the desktop and Android email client from Mozilla, just got an update to version 135. This is the start of another transition for Thunderbird: monthly major releases by default, matching the Firefox web browser.

Mozilla Thunderbird is available in different release channels, just like the Firefox web browser, giving people more choices in features vs. stability. The default download on the Thunderbird website until now, and the version distributed in some package managers, is the Extended Support Release (ESR) channel that is updated annually. Since version 124, there has also been a Release channel that receives monthly updates with new features and improvements.

Thunderbird is aiming to move more people to the Stable branch over the coming months, so they can get new features and bug fixes every month instead of once a year. Firefox 136 started officially rolling out today, and now we also have Thunderbird 136.

What’s New in Thunderbird 136

There aren’t any game-changing new features in Thunderbird 136, but if you’re switching from the ESR branch, there are a few helpful changes. The dark reader in the message pane has been improved, and messages are now automatically adapted to dark mode—if something looks broken, there’s a toggle in the header to turn it off.

The update also adds a settings switch to control message threading/sorting order across Thunderbird. There are many bug fixes too, including ones affecting attachments, email folders, notifications for IMAP accounts, and security issues.

Each monthly Stable release will have a similar assortment of bug fixes and improvements, so there will be more reasons to move from ESR as the feature divide grows. If you prefer to wait and not worry about anything breaking, you can just stay on the ESR channel, and these improvements will show up in the next major ESR update.

Thunderbird 136 requires Windows 10 or later, macOS 10.15 or later, or desktop Linux with GTK+ 3.14 or higher. Those are the same system requirements as the current ESR update, Thunderbird 128. The last update for Windows 7 and 8 was Thunderbird 115, back in 2023.

Moving From ESR to Stable

The Thunderbird website now lists the standard Release channel as the default download. The downloads available from the Windows Store, Snap, Flatpak, and other sources are still using the ESR. Mozilla previously said it plans to use in-app notifications and other features to encourage people to switch to the Stable channel, but existing ESR users will not be automatically migrated.

The Thunderbird team said in a February blog post, “One of our goals for 2025 is to increase daily active installations on the release channel to at least 20% of the total installations. At last check, we had 29,543 daily active installations on the release channel, compared to 20,918 on beta, and 5,941 on daily. The release channel installations currently account for 0.27% of the 10,784,551 total active installations tracked on stats.thunderbird.net.”

If you have a Windows or Mac computer, you can download the latest Stable version from the Thunderbird website, then install the application to the same directory where the ESR version is installed. If you’re on Linux, the process is much more complicated, and probably not worth it right now. Waiting for a simple Flatpak or Snap version is probably the right move.

Source: Thunderbird Blog (1, 2), Thunderbird Release Notes