Zen Browser Review: A Firefox Fork With Vertical Tabs and No Default AI

Short Answer: Zen Browser Review

Zen Browser is a capable desktop choice if you want vertical tabs without artificial intelligence (AI) chatbots active by default. At Ban the Bots, we track how browsers deploy automated features and how easily readers can disable them. Zen Browser stands out because it takes the Gecko rendering engine from Mozilla Firefox and strips out both standard data collection and default AI integrations. The project hosts its code in the zen-browser/desktop repository on GitHub under the Mozilla Public License 2.0 (MPL-2.0).

The software appeals directly to people moving away from Chromium browsers or seeking a modular layout similar to Arc. You get workspaces, split views, and compact sidebars out of the box. You also avoid the commercial pressure found in mainstream browsers that push cloud assistants into your workflow. If you want an overview of projects with similar design goals, read our guide to browsers without AI.

The main caveat is software maturity. Zen Browser is explicitly labeled as beta software, with release 1.22.2b tracking Firefox 156.0. It lacks mobile companion applications for Android and iOS, and its own updater shipped one high-severity flaw in a past release. It works well as a customizable secondary browser or a daily driver for technical users, but it cannot yet replace the stability of upstream Firefox for general audiences.

Zen Browser at a Glance

Zen Browser combines a Firefox rendering foundation with vertical tab management, stripped telemetry, and disabled machine learning preferences. The table below compares Zen Browser against Mozilla Firefox, Brave, and Arc across core technical specifications, default settings, and funding models.

What Zen Browser Changes About Firefox

Zen Browser modifies standard Firefox by replacing the horizontal tab bar with Workspaces and vertical navigation while removing default telemetry collection. The project describes itself on its homepage as a Firefox-based browser focused on productivity. The interface introduces Workspaces to group related tabs, Compact Mode to hide the sidebar when not in use, Glance for rapid page previews, and Split View to place two active tabs side by side. Users can also install visual themes and layout adjustments through Zen Mods.

Privacy changes go deeper than the visual theme. According to the Zen Browser privacy policy, the team does not collect telemetry data or crash reports. The developers state that Zen has stripped out the telemetry built into Mozilla Firefox. The policy notes that third-party code might contain overlooked reporting calls, points users to the Mozilla Firefox Privacy Notice for underlying components, and explains that default connections for software updates and add-on metadata can be disabled through advanced configuration in about:config.

The project follows a fast update schedule to maintain parity with upstream security patches. Zen documents on its security page that it updates Zen to the latest Firefox release within 0 to 72 hours. To achieve this turnaround, the team tests Firefox Release Candidate (RC) builds roughly one week before Mozilla ships them to stable channels. The latest release, version 1.22.2b published on September 16, 2026, updated the browser to Firefox 156.0. The version scheme uses major, minor, and patch numbers followed by a "b" suffix to indicate its beta status. Pre-release testing takes place on a dedicated channel named Twilight, which tracks upcoming Firefox RC packages.

Does Zen Browser Have AI Features Enabled?

Zen Browser disables upstream Firefox artificial intelligence chat features by default through configuration preferences set in early 2025. When Mozilla introduced an on-by-default sidebar chatbot in Firefox 136, users raised concerns about automated tools entering an open-source browser. On March 9, 2025, a contributor opened GitHub discussion #6236 asking the Zen team to deactivate the feature. Contributor noaione answered that a fix was already prepared for the Twilight channel.

Later that day, the team merged pull request #6244 titled "Disable AI chat stuff by default". This change explicitly set five internal preferences to false in the default configuration profile:

• browser.ml.chat.enabled
• browser.ml.chat.shortcuts
• browser.ml.chat.shortcuts.custom
• browser.ml.chat.sidebar
• browser.ml.enable

These settings deactivate the machine learning pipeline, remove the sidebar chatbot panel, and suppress keyboard shortcuts associated with AI prompts. Zen developers confirmed in the pull request discussion that they have no plans to lock these preferences. Users who choose to use Mozilla's machine learning tools can navigate to about:config and toggle any of the five preferences back to true.

Upstream developments added another layer to this topic. On February 24, 2026, Mozilla released Firefox 148, which introduced a dedicated AI Controls section in the desktop settings interface. As explained on the Mozilla blog, this panel features a master switch named "Block AI enhancements". Mozilla states that toggling this control blocks website translations, image descriptions in Portable Document Format (PDF) files, automated tab grouping, link previews, and the sidebar chatbot, while also suppressing prompts for future AI additions.

Whether Zen Browser surfaces this upstream AI Controls panel directly in its custom preferences menu is not documented. A review of the Zen Browser release notes reveals no mention of AI controls, machine learning preferences, or the Block AI toggle. Open your local Settings menu to confirm whether the panel is visible, or inspect about:config directly to verify your machine learning preferences.

Is Zen Browser Safe to Use?

Zen Browser is reasonably safe for general browsing, but its beta status and independent release pipeline introduce risks that mainstream browsers avoid. Because Zen is maintained by a small group of independent contributors, the build and update infrastructure does not match the formal testing scale of Mozilla or Google. Users must weigh the benefits of custom productivity features against the realities of experimental software.

A serious update vulnerability demonstrated this tradeoff in early 2026. On April 24, 2026, the project published GitHub security advisory GHSA-qpj9-m8jc-mw6q with a Common Vulnerability Scoring System (CVSS) score of 8.0 High. The advisory reported that the Zen Browser Mozilla Archive (MAR) updater shipped with signature verification removed, causing the browser to accept unsigned update archives. The flaw affected all versions through 1.19.8b. The developers patched the issue in release 1.19.9b by restoring signature checks.

The project provides compiled binaries on its download page, supporting macOS Universal packages, Windows standard and ARM64 installers, and Linux x86_64 and aarch64 archives alongside Flathub builds. Official releases on the GitHub releases page publish SHA256 checksums for every build artifact. However, the release documentation does not state whether builds carry commercial platform code signing or notarization certificates.

Why There Is No Zen Browser for Android or iOS

Zen Browser does not offer mobile applications for Android or iOS devices. The download page lists macOS, Windows and Linux builds only.

The official stance comes directly from the Zen Browser FAQ. The developers state: "At the moment, our team does not have the time or resources to develop Android or iOS versions of Zen Browser". The team also points out an interface design limitation: vertical tab bars and multi-workspace panels do not translate effectively to narrow smartphone screens.

This absence creates practical friction if you rely on a single browser ecosystem. You can run a different browser on your phone, but you will not have Zen's workspaces, glance previews, or custom mod layouts on your phone. If cross-device synchronization between matching desktop and mobile clients is an absolute requirement, look at our comparison in Brave vs Firefox to see how fully supported mobile ecosystems operate.

Zen Browser vs Firefox

Comparing Zen Browser to upstream Mozilla Firefox highlights a tradeoff between interface customization and long-term organizational backing. Firefox is developed by Mozilla. Zen Browser is a fork maintained by a small team and funded by donations and corporate sponsors such as BlackSmith, Crowdin, and Tuta, alongside individual supporters on Patreon and Ko-fi as listed in the Zen FAQ.

The interface marks the most obvious departure. Zen Browser integrates vertical tabs, workspaces, and window splitting into the core interface. Users who want modern productivity views get them immediately upon installing Zen.

Data handling and artificial intelligence policies also diverge. Firefox ships with the telemetry that Zen's privacy policy says it stripped out. Firefox 148 also added an AI Controls panel with a master switch, but it still ships active machine learning integrations like translation and PDF accessibility tools. Zen Browser strips out Mozilla's telemetry pipeline entirely and turns off all five machine learning chat preferences by default. For readers investigating alternative Firefox derivatives, our guide to LibreWolf vs Firefox examines another build focused on tracker removal, while Waterfox vs Firefox explores a different balance of performance and customization.

Zen Browser vs Brave

Zen Browser and Brave approach browser independence from opposing software engines and contrasting artificial intelligence strategies. Brave was founded in 2015 by Brendan Eich and Brian Bondy at Brave Software in San Francisco. Brave is built on Google's Chromium engine, while Zen Browser relies on Mozilla's Gecko codebase. Choosing Zen helps prevent Chromium from establishing a total monopoly over web standards.

The two browsers handle content blocking differently. Brave includes Brave Shields, which blocks third-party advertisements and cross-site trackers by default without requiring additional extensions. Zen Browser removes Mozilla telemetry out of the box, but it leaves standard web advertisements untouched unless you install an ad-blocking extension from the Firefox Add-ons repository.

Their positions on artificial intelligence present an even sharper contrast. Brave builds its own AI assistant, Brave Leo, into the sidebar and toolbar on macOS, Windows, Linux, Android and iOS. Leo has a free tier and a premium tier. In contrast, Zen Browser contains no proprietary AI services, disables upstream Firefox AI chat preferences by default, and sells no machine learning upgrades. If you want to see how Brave compares to other Chromium options, read our analysis of Vivaldi vs Brave.

Zen Browser vs Arc

Zen Browser has attracted former Arc users because both browsers prioritize vertical navigation, but Arc has shifted into a maintenance phase. On May 26, 2025, The Browser Company published a letter to Arc members announcing that The Browser Company was not actively developing the core product experience like it used to. The team redirected its engineering resources toward an AI-centered browser called Dia, while keeping Arc on routine Chromium upgrades, security patches, and bug maintenance.

Arc is a Chromium browser whose leadership chose to build around AI. In the May 2025 letter, the chief executive officer stated that the initial rollout of Arc Max was too cautious. As outlined on the Arc Max page, the browser offers features like 5 Second Previews, ChatGPT in the Command Bar, Tidy Tab Titles, and Tidy Downloads. Arc users must opt into Max features, but The Browser Company's product trajectory points toward heavier AI integration.

Zen Browser provides the vertical tab layout, split screens, and workspaces Arc users know, but it does so on an open-source Firefox foundation. Zen collects no browsing telemetry and turns the Firefox machine learning preferences off. For people who liked Arc's interface but rejected its AI direction, Zen is the closest desktop match. For more options outside the Google ecosystem, consult our list of Chrome alternatives.

Who Should Not Use Zen Browser

Zen Browser is a poor choice for anyone requiring mobile synchronization, corporate deployment guarantees, or fully settled software. If your workflow depends on opening tabs on your desktop and accessing them immediately on an Android phone or iPhone, Zen cannot perform that task. The development team has confirmed on their FAQ page that they lack the resources to build mobile applications, meaning mobile users must run a completely different browser on handheld devices.

Enterprise workers and corporate environments should also avoid Zen Browser. The software carries an explicit beta label, release assets do not publish enterprise notarization profiles, and past security advisories like GHSA-qpj9-m8jc-mw6q demonstrate that updater mechanisms can introduce vulnerabilities. System administrators needing signed Microsoft Installer packages, centralized group policies, or commercial service level agreements should deploy upstream Firefox instead.

Finally, users who want an out-of-the-box ad blocker should look elsewhere. While Zen strips internal Mozilla telemetry, it does not package an ad-blocking engine like Brave Shields. If you do not want to configure third-party extensions to block adverts, Brave, with Shields on by default, will require less manual setup.

What Would Change Our Verdict

Our assessment of Zen Browser would change if the project dropped its beta designation, added mobile support, or altered its default stance on artificial intelligence. If the development team were to integrate commercial AI assistants into the sidebar, monetize automated chat features, or enable upstream machine learning preferences by default, we would remove our recommendation for privacy-conscious users.

A breakdown in update reliability would also alter our position. Zen's primary security defense is matching Firefox releases within 0 to 72 hours. If that update window begins to stretch past several weeks, users would remain exposed to known web engine vulnerabilities, making the software unsafe for daily web traffic.

Conversely, if the team successfully releases an audited, stable version with an optional mobile companion that synchronizes data without third-party cloud servers, Zen Browser would rank among the best overall desktop tools available. To test the browser yourself, download the installer for your operating system directly from the official download page and verify the package checksum before first launch.

Frequently Asked Questions

Is Zen Browser safe?

Zen Browser is reasonably safe for personal daily use, but it is currently in beta and carries risks common to smaller open-source projects. In April 2026, security advisory GHSA-qpj9-m8jc-mw6q reported an updater vulnerability in versions through 1.19.8b that allowed unsigned update packages. The team resolved the issue in version 1.19.9b. Official release files include SHA256 checksums on GitHub, and the team updates the browser to track upstream Firefox security patches within 0 to 72 hours.

Does Zen Browser have AI features?

Zen Browser does not ship active AI features by default. When Mozilla introduced an experimental sidebar chatbot to Firefox, the Zen team merged pull request #6244 on March 9, 2025, which disabled five machine learning preferences including browser.ml.enable and browser.ml.chat.enabled. These preferences remain accessible in about:config if users choose to turn them back on. Zen's release notes make no mention of custom AI tools.

Is Zen Browser based on Firefox or Chromium?

Zen Browser is based on Mozilla Firefox and uses the Gecko rendering engine. Version 1.22.2b runs on Firefox 156.0. Choosing Zen allows users to support an independent rendering engine outside the Chromium ecosystem dominated by Google.

Is there a Zen Browser for Android?

There is no official Zen Browser application for Android or iOS. The official FAQ explains that the development team lacks the time and resources to maintain mobile ports. The developers also note that vertical tab layouts do not translate effectively to smartphone screens. Any app on mobile stores claiming to be Zen Browser is unauthorized.

Is Zen Browser open source?

Zen Browser is completely open source. The desktop application source code is published in the zen-browser/desktop repository on GitHub and is licensed under the Mozilla Public License 2.0 (MPL-2.0). Anyone can inspect the code, review pull requests, or compile the software independently.

How is Zen Browser funded?

Zen Browser is funded through donations and corporate sponsorships rather than advertising or user tracking. The project lists corporate sponsors including BlackSmith, Crowdin, and Tuta on its homepage, alongside community funding through Patreon and Ko-fi linked on its documentation FAQ.

Sources

• Zen Browser Desktop Repository (GitHub)
• Zen Browser Official Homepage
• Zen Browser Release Notes
• Zen Browser Download Portal
• Zen Browser Frequently Asked Questions
• Zen Browser Privacy Policy
• Zen Browser Security Documentation
• GitHub Pull Request #6244: Disable AI chat stuff by default
• GitHub Discussion #6236: Disable Firefox AI chatbot
• GitHub Security Advisory GHSA-qpj9-m8jc-mw6q
• Zen Browser Release Assets (GitHub)
• Mozilla Firefox 148.0 Release Notes
• Mozilla Blog: AI Controls in Firefox
• The Browser Company: Letter to Arc Members (May 2025)
• Arc Max Features Overview
• Brave Software Company Information
• Brave Leo Smart AI Assistant