fidzu : thunderbird

Over a year ago, we introduced an updated version of the sidebar that offers easy access to multiple tools - bookmarks, history, tabs from other devices, and a selection of chatbots - all in one place. As the new version has gained popularity and we plan our future work, we have made a decision to retire the older version in 2026.

Old sidebar version

Updated sidebar version

We know that changes like this can be disruptive - especially when they affect established workflows you rely on every day. While use of the older version has been declining, it remains a familiar and convenient tool for many - especially long-time Firefox users who have built workflows around it.

Unfortunately, supporting two versions means dividing the time and attention of a very small team. By focusing on a single updated version, we can fix issues more quickly, incorporate feedback more efficiently, and deliver new features more consistently for everyone. For these reasons, in 2026, we will focus on improving the updated sidebar to provide many of the conveniences of the older version, then transition everyone to the updated version.

Here's what to expect:

• Starting with Firefox Nightly 148, we have turned on the new sidebar by default for Nightly users. The new default will remain Nightly-only for a few releases to allow us to implement planned improvements, existing community requests, and collect additional feedback.
• In Q2 2026, all users of the older version in release will be migrated to the updated sidebar. After the switch, for a period of time, we will keep the option to return to the older version to support folks who may be affected by bugs we fail to discover during Nightly testing. During this period, you will still be able to temporarily switch back to the old sidebar by going to Firefox Settings > General > Browser Layout and unchecking the Show sidebar option.
• In Q3 2026, we will fully retire the original sidebar and remove the associated pref as we complete the transition.

Our goal is to make our transition plans transparent and implement suggested improvements that are feasible within the new interaction model, while preserving the speed and flexibility that long-time sidebar users value. Several implemented and planned improvements to the updated sidebar were informed by your feedback, and we expect that to continue throughout the transition:

• Hide sidebar launcher on panel close if it was hidden prior to panel open (planned)
• Full screen mode doesn't hide sidebar tools (planned)

If you'd like to share what functionality you've been missing in the new sidebar and what challenges you've experienced when you tried to adopt it, please share your thoughts in this Mozilla Connect thread or file a bug in Bugzilla's Sidebar component, so your feedback can continue shaping Firefox.

13 Jan 2026 10:57pm GMT

WebDriver is a remote control interface that enables introspection and control of user agents. As such it can help developers to verify that their websites are working and performing well with all major browsers. The protocol is standardized by the W3C and consists of two separate specifications: WebDriver classic (HTTP) and the new WebDriver BiDi (Bi-Directional).

This newsletter gives an overview of the work we've done as part of the Firefox 147 release cycle.

Contributions

Firefox is an open source project, and we are always happy to receive external code contributions to our WebDriver implementation. We want to give special thanks to everyone who filed issues, bugs and submitted patches.

In Firefox 147, two WebDriver bugs were fixed by contributors:

• Sajid Anwar fixed an issue with browsingContext.navigate which could return a payload with an incorrect URL.
• Khalid AlHaddad added new WebDriver classic tests for the "Get All Cookies" command.

WebDriver code is written in JavaScript, Python, and Rust so any web developer can contribute! Read how to setup the work environment and check the list of mentored issues for Marionette, or the list of mentored JavaScript bugs for WebDriver BiDi. Join our chatroom if you need any help to get started!

General

• Fixed the new session response to include the required setWindowRect property.

WebDriver BiDi

• Implemented the input.fileDialogOpened event, which is emitted whenever a file picker is triggered by the content page, for instance after clicking on an input with type="file".
• Implemented the emulation.setScreenSettingsOverride command to allow clients to emulate the screen dimensions for a list of browsing contexts or user contexts.
• Fixed an issue where browsingContext.navigate with wait=none didn't always contain the real target URL.
• Updated script.evaluate and script.callFunction to bypass Content Security Policy (CSP).
• Fixed missing script.realmCreated event for new browsing contexts created via window.open.
• Updated emulation.setLocaleOverride to override the Accept-Language header.
• Updated emulation.setLocaleOverride to throw an error when called with the locale argument equal to undefined.

Marionette

• Fixed JSON serialization of Chrome Windows.

13 Jan 2026 5:18pm GMT

Highlights

• Windows tests now start twice as fast! Thanks to improvements in how we provision Windows machines in the cloud, Yaraslau Kurmyza and RelOps cut startup delays dramatically. Since December 9th, it now takes 3 times less time to get a Windows worker ready, which has reduced Windows test wait times by half.

• AGENTS.md and CLAUDE.md were added to the Firefox repository.

• Calixte implemented most of the backend functionality to support reorganizing pages, splitting and merging PDFs.

Contributors

• Arthur Silber

• Tim van der Meij

Detailed Project Updates

AI for Development

• AGENTS.md and CLAUDE.md were added to the Firefox repository.

• An AI coding policy was published in the Firefox source docs.

• Suhaib Mujahid built an MCP server to facilitate the integration of AI assistants with the Firefox development tooling, whichIt enables AI assistants to search using Searchfox, read Bugzilla bugs and Phabricator revisions, access Firefox source documentation, and streamline patch review workflows.

• Suhaib Mujahid extended the test selection system to work with local changes, enabling AI assistants to leverage our ML-based test selection for automatic identification of relevant tests, allowing them to iterate faster during development.

• Suhaib Mujahid implemented improvements to the Review Helper tool to improve the accuracy of suggested review comments.

Bugzilla

• Thanks to Kohei, when a user enters a comment on the show bug page, it will update the page instantly without a reload. (see Bug 1993761)

• Thanks to external contributor Logan Rosen for updating Bugzilla to use a newer version of libcmark-gfm which will solve some issues with rendering of Markdown in comments. (see Bug 1802047)

Build System and Mach Environment

• The dependency on Makefile.in has been reduced. The path is still long, but it's getting a bit closer (see Bug 847009 )

• Faster configure step thanks to faster warning flag checks (see Bug 1985940 )

• Alex Hochheiden upgraded the JavaScript minifier from jsmin to Terser and enabled minification for pdf.js to improve loading performance.

• Alex Hochheiden optimized glean-gradle-plugin and NimbusGradlePlugin configuration. Gained ~10s configuration time speedup and ~200MB disk space saved.

Firefox-CI, Taskcluster and Treeherder

• Your CI tasks are going to start faster! After many changes of different sizes, the entire Release Engineering team is proud to announce that the decision task is as fast as the best record from 2019 and even faster than ever before on autoland. We intend to beat the record on try with a few more patches close to landing.

• Windows tests now start twice as fast! Thanks to improvements in how we provision Windows machines in the cloud, Yaraslau Kurmyza and RelOps cut startup delays dramatically. Since December 9th, it now takes 3 times less time to get a Windows worker ready, which has reduced Windows test wait times by half.

• Ever wondered if your try-push scheduled the right tasks? Treeherder now shows unscheduled jobs too. Hit s to toggle visibility and cut down CI guesswork!

• Abhishek Madan made various performance improvements to the decision tasks totalling to around 25% improvement

• Abhishek Madan switched Decision tasks to a faster worker-type

• Andrew Halberstadt kicked off the CI migration from hg.mozilla.org → Github, implementing:

  • Shallow clone support in run-task

  • A dedicated Decision task that responds to Github events

• Ben Hearsum added support for outputting the relationships between taskgraph kinds as Mermaid diagrams, making it easier to visualize the relationships between tasks.

• Matt Boris added the finishing touches on D2G (Docker Worker to Generic Worker translation layer) to enable Julien Cristau to begin rolling changes out to L3 pools.

Lint, Static Analysis and Code Coverage

• New include linter through mach lint -lincludes . Unused MFBT and standard C++ headers are reported.

• Alex Hochheiden fixed many lint warnings and upgraded them to errors.

• Alex Hochheiden replaced black with ruff-format.

PDF.js

• Calixte implemented most of the backend functionality to support reorganizing pages, splitting and merging PDFs.

• Calixte added support for tagged math in PDFs in order to make math content accessible.

• Tim van der Meij helped with maintenance and improvements to pdf.js CI, like using OICD trusted publishing.

• Aditi made it so we serialize pattern data into ArrayBuffer, paving the way for moving pdf.js rendering in worker threads.

• Arthur Silber improved text rendering performance by skipping unnecessary pattern calculations, leading to up to 84% reduction in pdfpaint time for some PDFs.

• Calixte added support for the pdfium jbig2 decoder compiled in wasm in order to replace the pure JS version.

Firefox Translations

• (Bug 1975487, 1994794, 1995403) Erik Nordin shipped significant improvements to the Translations experience when translating web pages between left-to-right and right-to-left languages.

• (Bug 1967758) Erik Nordin improved the algorithm for page-language detection, centralizing the behavior in the parent process, instead of creating a separate language detector instance per content process.

• Evgeny Pavlov trained Chinese Traditional

• Sergio Ortiz Rojas trained English to Vietnamese

• Evgeny Pavlov created new evaluation dashboards with expanded metrics, datasets and LLM explanations

• Evgeny Pavlov migrated the model registry from Github to Google Cloud Storage with the updated UI (new models JSON)

Phabricator, moz-phab, and Lando

• Zeid and Olivier implemented various changes in Lando to support the GitHub pull request pilot project.

• Zeid added support for short hash when querying git2hg commit maps in Lando.

• Connor Sheehan implemented uplift requests as background jobs, providing many improvements to the uplift request workflow in Lando:

  • Merge conflict detection at job completion time, instead of at landing time.

  • Uplift to multiple trains at once, with failure notification emails that provide step-by-step commands to resolve the conflict and re-submit.

  • Uplift assessment form linking workflow to avoid re-submitting the same form when manually resolving merge conflicts for an uplift.

• Connor Sheehan made it possible to select individual commits in the stack for uplift, instead of always uplifting the parent commits for a given revision.

• Connor Sheehan added a new uplift assessment linking view and hooked it into moz-phab uplift, removing a few steps between submitting an uplift request and opening the form for submission or linking to the new request.

• moz-phab had several new releases.

  • Mathew Hodson restored the --upstream argument to moz-phab submit.

  • Jujutsu support saw improvements to moz-phab patch, better handling of working copy changes and a minimum jj version bump to 0.33.

  • moz-phab uplift saw a few changes to enable better integration with the Lando-side changes.

  • See the release notes here:

    • https://discourse.mozilla.org/t/mozphab-2-6-0-released/146283

    • https://discourse.mozilla.org/t/mozphab-2-7-0-released/146293

    • https://discourse.mozilla.org/t/mozphab-2-7-1-released/146295

    • https://discourse.mozilla.org/t/mozphab-2-7-2-released/146339

    • https://discourse.mozilla.org/t/mozphab-2-8-0-released/146434

    • https://discourse.mozilla.org/t/mozphab-2-8-1-released/146774

Version Control

• Connor Sheehan added clonebundle buckets in the us-east1 GCP region to improve clone times in CI.

• Julien Cristau added the new tags Mercurial branches to mozilla-unified.

• Julien Cristau and Olivier Mehani took steps to reduce OOM issues on the hg push server.

• Julien Cristau resolved a Kafka issue by pruning try heads and resolving issues with try heads alerting, and Greg Cox increased the storage in Kafka in support of the mitigation.

• Greg Cox implemented staggered auto-updating with reboots on the load balancers in front of hg.mozilla.org.

Thanks for reading and see you next month!

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13 Jan 2026 1:51pm GMT