Firefox 135: Major Updates and New Features in the Latest Release

Firefox 135 brings a series of exciting new features and improvements aimed at enhancing the user experience for both developers and everyday users. As this version rolls out, it includes everything from visual updates and API enhancements to performance improvements, offering a more streamlined and efficient browsing experience. Below, we will summarize the key highlights, followed by a deeper dive into what these changes mean for developers, users, and the wider Firefox ecosystem.

Key Highlights of Firefox 135 Release

Sidebar and Vertical Tabs: These long-awaited features have been moved from Nightly Labs to the main settings interface in about:preferences, giving users more control over their browser’s layout.
Manifest V3 UserScripts API: This API has now been enabled by default for desktop builds and is set to debut in Firefox 136, allowing for greater flexibility and functionality for developers.
New WebExtension API for Shortcuts: The of the browser.commands.openShortcutsSettings method allows extensions to guide users directly to the keyboard shortcuts management interface, improving accessibility and customization.
Crash UI for Bug Reporting: Firefox has added improved crash reporting UI, making it easier for users to report issues on Bugzilla.
New Contributions from Developers: Several notable contributions have been made, including new features in the Add-ons Manager and various bug fixes that enhance the overall performance of the browser.

What’s New for Add-ons and Web Extensions

Add-ons Manager Improvements: Bugs related to the mouse selection of text in the extension options page and a memory leak in the install dialog were fixed. Users will also soon be able to undo theme installations directly from notifications.
WebExtensions Framework: Firefox 136 introduces a new background.preferred_environment manifest property to help developers ensure that extensions are compatible across different browsers, specifically enabling cross-browser compatibility between Chrome and Firefox.
DevTools Enhancements: Multiple improvements were made to the Firefox Developer Tools, including faster performance for the Netmonitor and improvements to JavaScript debugging and WASM (WebAssembly) support.
WebDriver BiDi Improvements: Several key improvements to WebDriver, including enhanced subscription commands and better crash report integration, make Firefox even more developer-friendly.

Performance, Stability, and Usability Improvements

New Tab Page Overhaul: Significant changes have been made to the New Tab page’s code, including memory optimizations and a redesign of top site stylings. These improvements are aimed at reducing memory usage and improving startup times, as well as enhancing the overall UI experience.
Search and Navigation: The new release focuses heavily on improving the search experience, including tweaks to the Unified Search Button and updates to the search configuration experiments. Also, AMP and Suggest Dismissal issues have been resolved.

What Undercode Says:

In-depth Analysis of Firefox 135 Features and Enhancements

The Firefox 135 release marks a major milestone in the ongoing efforts to refine both user and developer experiences. The addition of Sidebar and Vertical Tabs as fully integrated features is a win for users who prefer a more organized browsing experience. This change not only improves accessibility but also enhances the browser’s adaptability to different screen sizes and user preferences. By making these options available in the main settings interface, Firefox ensures a smoother user experience for those looking to customize their layout.

The Manifest V3 UserScripts API is another noteworthy update. This new API, which is being rolled out in Firefox 136, provides developers with a more robust way to control user scripts. Manifest V3 represents a significant shift towards improving performance and security, and its integration into Firefox’s ecosystem is a step toward keeping up with industry standards.

One of the most exciting features from a development perspective is the new WebExtension API for Shortcuts. This allows extensions to interact with the keyboard shortcuts management interface, an upgrade that could improve both accessibility and ease of use. The ability to direct users to specific UI sections via an API is a subtle but powerful feature, especially for power users who rely on browser extensions to streamline their workflows.

Performance improvements, especially on the New Tab Page and Netmonitor, also deserve a mention. These changes not only make Firefox snappier but also improve memory usage—an essential consideration for users running multiple tabs or applications. The optimization in handling large data URIs in Netmonitor and WASM debugging is a big plus for developers who deal with complex web applications.

The Add-ons Manager updates are also worth noting. Fixes for mouse selection issues and leaks in the install dialog contribute to a smoother experience for users managing extensions. The ability to undo theme installations from the notification popup is a small, yet highly practical enhancement that will likely be appreciated by many.

Finally, the performance gains resulting from refactoring code for the New Tab page are a testament to Firefox’s ongoing commitment to efficiency. Faster startup times and reduced memory usage are crucial improvements, especially in an era where many users run multiple resource-intensive tabs simultaneously.

Fact Checker Results:

Sidebar and Vertical Tabs: These features have indeed been added to Firefox preferences as expected, confirming the rollout to the stable release.
Manifest V3 UserScripts API: This API has been enabled by default and is confirmed to be included in Firefox 136.
WebExtension API for Shortcuts: The new method browser.commands.openShortcutsSettings is functional and integrated into the current release as reported.

References:

Reported By: https://blog.nightly.mozilla.org/2025/03/11/turn-tabs-to-their-side-these-weeks-in-firefox-issue-177/
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