Thunderbird 155 is now available as the latest version of Mozilla's email client, incorporating significant improvements to account configuration, OAuth authentication, and compatibility with custom mail servers. This release also comes at a time of change for the project, which, starting in September, began adopting a much faster development cycle, with new stable versions released every two weeks.
The new version continues Thunderbird's evolution toward a more flexible email client, better suited to modern services. Key new features include custom OAuth support for IMAP and SMTP servers, along with new capabilities for extensions to modify the authentication providers used by specific domains. Numerous bug fixes, stability improvements, and security updates are also included.
Thunderbird 155 improves OAuth authentication
The main new feature in Thunderbird 155 is related to authentication. The new version includes support for configuring custom OAuth providers on IMAP and SMTP servers , which is especially useful for organizations and email providers that use their own authentication systems.
Until now, account configuration in Thunderbird relied heavily on the OAuth providers integrated into the client itself. With the new version, the system is more flexible and allows you to define specific configurations for servers and domains that don't necessarily use the default providers.
The improvement is especially relevant in business, educational or managed environments, where the mail server may use a different authentication infrastructure than that used by large commercial services.
Extensions can modify authentication providers
Thunderbird 155 also expands the capabilities of OAuth-related extensions. Add-ons can now override built-in configurations for specific domains and hostnames.
This allows the development of extensions capable of providing specific authentication mechanisms without necessarily having to wait for each provider to be incorporated directly into the main Thunderbird code.
The change also improves Thunderbird's ability to adapt to specialized services. Instead of treating all mail servers the same way, the client can use different configurations depending on the domain or server it's working with.
Manual account setup is more comprehensive
Another improvement relates to the manual account setup process. Thunderbird 155 simplifies this workflow and adds support for Exchange, Microsoft Graph, and custom OAuth within the same experience.
This is especially important because Thunderbird is progressively expanding its support for Microsoft Exchange. The project has been working for months on integrating Microsoft Graph to gradually replace various components of the older EWS-based implementation.
Thunderbird 154 already enabled Microsoft Graph for Microsoft 365, while version 155 continues to develop the necessary infrastructure to configure and authenticate these types of accounts more easily.
OAuth incorporates PKCE and authentication via external browser.
Thunderbird 155 also adds specific fields to configure PKCE and the use of an external browser within the custom OAuth parameters.
PKCE, short for Proof Key for Code Exchange, adds a layer of protection to the authorization flow and is especially geared towards customers who cannot securely maintain customer secrets. Its use has become standard practice in modern applications that employ OAuth.
External browser support also aligns with the evolution Thunderbird had already begun in previous versions. Starting with Thunderbird 153, OAuth login for email accounts can be opened in the system's default browser, instead of performing the entire process within the application itself.
S/MIME improvements
The new version also includes an improvement for users of S/MIME . The corresponding information box now displays additional details about message signatures and encryption.
The information that can be consulted includes the algorithm used for the signature, the digest algorithm, the encryption algorithm, and the key size.
This information allows for more precise verification of the cryptographic characteristics of a signed or encrypted message and offers greater transparency to users who need to verify how a communication has been protected.
More fixes for IMAP, Exchange, and calendars
Thunderbird 155 comes with a long list of fixes. These include solutions to problems related to IMAP tag synchronization, global message search, and attachment filters.
Several issues related to Exchange and CalDAV have also been fixed. For example, deleting an Exchange account could leave the outgoing server settings in place, while certain CalDAV calendars could stop synchronizing their events altogether when using offline mode.
The client also fixes an issue that could cause the silent loss of changes made to cached ICS files when Thunderbird reconnected.
Numerous stability issues have also been corrected in Thunderbird 155
The update includes several bug fixes that could cause the program to crash unexpectedly. Among the issues resolved are various errors related to database memory management and loading email folders.
A crash that could occur during Thunderbird shutdown while the client waited for an OAuth2 password to be entered has also been fixed.
These changes may be less visible than new features, but they are especially important in an email client that remains open for long periods and continuously manages messages, folders, calendars, contacts, and local databases.
With Thunderbird 155 a new stage begins with more frequent versions
The release of Thunderbird 155 also has an important peculiarity: it is part of the new accelerated release schedule adopted by Mozilla and Thunderbird.
The project has decided to move from a four-week cycle to one of approximately two weeks for desktop versions. Thunderbird 155 is one of the first versions to benefit from this change, which aims to allow completed improvements to reach users more quickly.
The team explained that this model is not intended to force developers to work at twice the speed. Features that aren't ready can continue development and wait for a later version, while improvements that are ready will have a better chance of reaching the stable branch quickly.
Smaller and more frequent versions
The new schedule means Thunderbird will have to get used to a series of releases with much shorter intervals. Instead of concentrating a large number of changes into a single monthly update, improvements can be spread across several smaller releases.
The strategy follows the previously announced change for Firefox, which has also begun experimenting with a two-week release cycle. Thunderbird also maintains synchronized numbering with Firefox, so Thunderbird 155 is based on Firefox 155.
For users, the updater's normal operation remains unchanged. New versions will continue to arrive via Thunderbird's update mechanisms or through the repositories of the various Linux distributions.
Thunderbird 155 reinforces its commitment to Exchange
The evolution of Exchange support remains one of Thunderbird's most important technical projects. The team has been working for months to expand compatibility with Microsoft Graph and bring it closer to the capabilities previously provided by EWS.
Thunderbird 155 benefits from this work by integrating Exchange and Graph into the manual account setup flow. Combining this support with custom OAuth allows the user to adapt to a wider range of enterprise configurations.
The project has also confirmed that work on Graph doesn't end with email. Once email compatibility is established, Thunderbird will continue developing other parts of the Exchange integration, including future calendar-related improvements.
A major update for the Mozilla email client
Thunderbird 155 represents an update focused primarily on authentication, account configuration, and compatibility with various email infrastructures. Custom OAuth support for IMAP and SMTP is arguably its most significant addition, while PKCE, external browser support, and extension improvements round out a major overhaul of this part of the program.
This version also includes S/MIME improvements, new administrator policies, and numerous fixes related to IMAP, Exchange, CalDAV, search, filters, and stability. All of this comes as Thunderbird begins a new phase of more frequent releases.
By moving to a cycle of approximately two weeks, the project aims to reduce the time between the completion of an improvement and its arrival to users. Thunderbird 155 is therefore more than just a maintenance update: it also demonstrates the new development strategy with which Mozilla's email client will approach the coming months.