Zen Browser 1.23.1b lands as a security patch on Firefox 157.0.1
The latest stable build of the Firefox fork polishes its new Library panel, fixes a couple of macOS quirks, and quietly pulls users onto Mozilla's latest security base.
Zen Browser just pushed 1.23.1b to its stable channel, and the version number tells you everything you need to know before you even read the notes. The trailing "b" marks this as a maintenance build rather than a feature drop. It landed Tuesday, Oct. 6, around 06:55 UTC, and it arrives directly on top of the major 1.23 release from Oct. 2.
Keep in mind the tiering. That "b" and the "Release build" label put this in Zen's production-ready stream, the same one that also goes by "Zen Stable Release." The experimental channels, like Twilight, are built against Firefox release candidates and are a different story entirely.
What's actually in 1.23.1b
Under the hood, the core upgrade is straightforward: Zen rides Firefox 157.0.1, which itself bundles Mozilla's most recent security and stability fixes. On top of that, the team layered in a handful of small refinements, most of them aimed at the new Library panel from 1.23.
The Library gains a closed-tab filter, so you can hunt down tabs you've shut without wading through your full browsing history. There's now a keyboard shortcut, Opt/Alt + Shift + L, to open the Library directly. And right-clicking the section tabs inside it gives you "Clear all downloads" and "Clear history" without extra menu diving.
The fixes are narrower but still useful. It corrects a styling bug where transparent sidebars rendered at an unexpectedly low opacity, which no doubt looked a bit off. It also repairs a macOS download bug that tripped up users when the browser asks where to save a file but can't reach the Downloads folder. Both are exactly the sort of thing you want squashed in a patch.
The security half is where the real reason for the build lives. Zen cites Mozilla advisory MFSA2026-104, announced the same day, which fixes CVE-2026-106016. That's a "moderate" impact mitigation bypass in the File Handling component, reported by Abdulrahman Alzahrani and tracked in Mozilla bug 2067465. Upgrading closes the gap.
Here's the part worth stating plainly: Zen re-packages Firefox, so its security posture tracks Mozilla's cadence rather than running an independent audit. It benefits from Mozilla's program but doesn't ship a fully separate engine. Zen's own notes defer to the Mozilla advisory for detail rather than publishing a separate write-up, which is standard practice for a Firefox-based fork.
The Firefox dependency
Why does Zen matter in the first place? The Browser Company announced in May 2025 that it would stop adding new features to Arc. Coverage immediately began casting Zen as the spiritual successor to that vision: a Firefox-based browser with tab organization, workspaces, and tiling, minus a corporate roadmap that could quietly stall.
The reception has been favorable. Liam Proven at The Register praised Zen for being free of Google code, offering more usable versions of Arc's features, and supporting older operating systems. Jack Wallen at ZDNet recommended it for improving Firefox's customization and tab management. Adam Conway at XDA called it a blend of the best of Floorp, Vivaldi, Firefox, and Chrome.
There is a known caveat he flagged: DRM-incompatible media was historically its main weakness. Zen has worked through these issues over time, and reviewers say early builds were rougher than what ships today.
The 1.23 line itself moves fast. 1.23b dropped Oct. 2 and introduced the Library as its headline feature, alongside a resizable sidebar, hardware AV1 decoding for WebRTC calls, and startup improvements when you've got a mountain of tabs open. 1.23.1b is basically the team stabilizing that feature rather than shipping more.
Look back a bit further and the pattern is obvious. 1.22.3b (Sept. 22) rode Firefox 156.0.1. 1.22b (Sept. 4) brought a "squircle" chrome redesign and a fresh onboarding flow. During active stretches Zen ships roughly every one to two weeks. And the engagement on this build reflects its patchy nature: the 1.23.1b post pulled in about 12 reactions on GitHub, versus roughly 60 for the major 1.23b release.
Head here to grab it from the GitHub releases page.
