Firefox 155 Starts Mozilla’s Two-Week Release Experiment

Firefox 155 reached stable users on September 1, 2026, the first desktop and Android build on Mozilla’s two-week release experiment. Firefox 156 is due September 15, with 157 on September 29.

Sylvestre Ledru, Mozilla’s director of engineering, framed the change in July as a trial, not a lock. The bet is that Gecko can share Chrome’s 14-day clock without rushing unfinished work, and September is the first month that bet has to clear three major trains.

Mozilla Is Betting Gecko Can Run on Chrome Time

Ledru told the dev-platform list on July 9, 2026, that Firefox Desktop and Android would move from a 4-week cadence to a two-week release cadence starting in September. He called it an experiment. The stated aims were more frequent chances to ship work that is already ready, a more predictable process, and less pressure to uplift fixes onto a branch that had already frozen.

This does not mean that all work needs to ship twice as fast. Work that is not ready should not be rushed, and features can still take the time they need to bake.

Sylvestre Ledru, Director of Engineering, Mozilla, dev-platform mailing list, July 9, 2026

Firefox 155 was pulled forward from September 15 to September 1, a 14-day shift that turned the last 4-week gap into the first 2-week one. Firefox 154 on August 18 was the final build on the old interval. Mozilla said it would watch quality, load, and user experience, then adjust if the new rhythm broke down.

That hedge matters because Firefox still ships an independent engine. Chrome and Edge can share Chromium’s branch cuts. Firefox has to cut Gecko, Android’s GeckoView, and the desktop product on the same Tuesday, with a smaller staff and a separate test grid.

The 12-Day Stretch When Every Major Browser Went Biweekly

The industry did not wait for Mozilla. Microsoft Edge 152 reached Stable on August 27, the first Edge major on a two-week cadence, while its Extended Stable channel stays on an 8-week cycle. Google had Chrome’s own two-week cycle on the calendar since March 3, 2026, and it executed on September 8 with Chrome 153 on Desktop, Android, and iOS. Chrome 154 is already in Beta, with Stable set for September 22.

From Edge 152 on August 27 to Chrome 153 on September 8 is 12 days. In that window, the three desktop browsers that still set the web’s default clock all left the 4-week standard they had shared since 2021. Chrome’s launch post named Microsoft, Mozilla, and Brave as moving with it.

THE FIRST TWO-WEEK TRAINS

Chrome’s public case is smaller releases, faster regression isolation, and a shorter gap between a fix landing in the tree and reaching Stable. Mozilla’s public case is almost the same list, written for a different engine. If the experiment holds, a ready Gecko fix that misses one train waits about two weeks instead of four. If it fails, Mozilla has already said it can lengthen the gap again. Reversing after Chrome and Edge have settled on 14 days would put Firefox back on the slow side of every web-platform comparison.

What Changed in Firefox 155 Besides the Calendar

The first two-week build was not an empty train. The Firefox 155.0 release notes date the offer to Release users as September 1, 2026, and they read like a normal major: a few visible features, a pile of platform work, and a security advisory.

WHAT 155 ACTUALLY SHIPPED

• Smart Window: The opt-in AI browsing mode is open to all users in the USA, Canada, and France, still on a progressive roll-out, and it needs a Mozilla account.
• Faster assistant: Mozilla says the assistant now returns Exa web-search citations in about one-fifth the prior time, with resume prompts for earlier chats.
• Tracker count: The address bar can show how many trackers Firefox blocked on the current page, and the number can be hidden without turning blocking off.
• Linux sleep: Firefox no longer stops a Linux machine from sleeping after a long browsing session, and text no longer vanishes in Microsoft 365 Word Online on some keyboard layouts.
• Controllers and containers: Nintendo Switch Pro pads map correctly in web games on Windows, and containers can be reordered in Settings.
• Translations: Built-in translation adds Marathi and Urdu.

Under the hood, Firefox 155 races IPv6 and IPv4 with Happy Eyeballs v3, can negotiate QUIC version 2, and lets the CSS attr() function feed any property, not only content. It also caps CSS nesting at 75 levels so extreme nesting cannot crash the parser. mailto: links now open only after an explicit user action. Captive-portal checks moved from detectportal.firefox.com to firefox-portal-detection.com, so office allowlists that still pin the old host will fail until someone updates them.

The same day, Mozilla published Foundation Security Advisory 2026-82 for Firefox 155, tagged high impact, and shipped matching ESR builds. Release-channel Firefox also dropped Windows 8.1 and earlier, plus macOS 10.14 and earlier; those machines are pointed at ESR. A 155.0.1 notes page followed on September 4, three days after 155.0. The first two-week major did not even last a week before a point release.

Nightly and Beta Now Get Half the Soak Time

Ledru’s line about not rushing work is the product promise. The calendar is the constraint. On the old 4-week pattern, Firefox 154 sat in Nightly from June 16 to July 22, 36 days, then in Beta from July 22 to August 18, 27 days. On the new pattern, Firefox 157 is listed with Nightly from August 27 to September 10, 14 days, and Beta from September 10 to September 29, 19 days.

SOAK TIME ON THE OLD TRAIN AND THE NEW ONE

Mozilla’s localization team spelled out the same squeeze on July 23, 2026. A string that used to have as little as three weeks between merge day and Release now has two. Pontoon deadlines for desktop and Android moved to a two-week lead starting August 12. Many strings still land earlier, behind flags, but the worst case is now 14 days of translation instead of 21.

That is also a shorter beta window for add-on authors. API changes, deprecations, and behavior tweaks arrive twice as often. Nightly and Beta are still the early-warning channels; they just do not sit still as long. The bet only works if each train really is about half as large. If 157 tries to carry a 154-sized diff through a 14-day Nightly, the “do not rush” rule collides with the calendar.

A Yearly ESR and a Two-Week Patch Train

Enterprise Firefox is not being asked to take a new UI every 14 days. Mozilla’s administrator docs still say new ESR versions arrive annually, with security and stability in the minor and patch numbers and no feature churn in between. Firefox 153 opened as ESR on July 21, 2026. That branch is slated for at least 15 months of updates, into late 2027. Firefox 115 ESR, the long tail for older Windows and macOS, is still listed through March 2027.

The patch clock is another matter. The public calendar lines ESR 153.2 up with Firefox 155 on September 1, 153.3 with 156 on September 15, and 153.4 with 157 on September 29. Git branches for esr153, esr140, and esr115 were still moving as of September 10. So the yearly ESR promise is about features. The security packets now ride the same Tuesday cadence as Release.

Admins who want the new major release every two weeks stay on Release and test in Beta. Admins who want a frozen feature set stay on ESR 153 and still take a patch every two weeks. Either way, the old four-week quiet period is gone. The portal-host change in 155 is a small preview of what a faster train does to managed networks: a setting that used to wait a month now shows up on the next other Tuesday.

Tails and Tor Browser Just Inherited the Same Clock

The first outside products to say the quiet part aloud were not Chrome clones. They were Gecko downstreams that exist because Firefox is not Chromium. On September 3, 2026, Tails 7.12 shipped with Tor Browser 15.0.21 and said Firefox’s new cadence was now theirs too. Tor Browser 15.0.21 itself went out on September 1, based on Firefox 140.15.0 ESR, with security fixes backported from Firefox 155.

Firefox is moving to a 2-week release cadence starting in September, and so Tor Browser and Tails are doing the same. Tails 7.12 is the first release on this new cadence.

Tails, version 7.12 notes, Tor Project blog, September 3, 2026

Tails 7.12 out: https://t.co/uU2an31pRk It's the first release on the new 2-week cadence of Firefox.

— Tails (@Tails_live) September 3, 2026

That is the part of the bet that does not show up in a desktop auto-update. A privacy OS and Tor Browser now plan around Mozilla’s Tuesdays. Faster upstream patches can close the gap between a Firefox fix and a Tails USB stick, which is the security case. It also means those projects inherit Mozilla’s miss if a two-week Gecko train is dirty. Tor Browser’s stable channel remains on ESR; its alpha work has already been pointed at Rapid Release. The 14-day clock is no longer a Firefox-only operations problem.

Firefox forks felt it in the same week. Floorp, which rebases on Mozilla’s desktop tree, shipped version 12.17.2 on September 5 and said it was rebuilt on Firefox 155. Anyone who packages Gecko used to absorb about a month of changes at a time. They now absorb a smaller delta twice as often, on someone else’s calendar. One reply in the Tails discussion also noted a practical split: a Tor Browser version number is not proof that a Tails image is current, because the two still ship as separate artifacts. The cadence matches; the checksums do not merge.

Firefox 156 Is Due on September 15

Mozilla’s calendar keeps every other Tuesday through Firefox 164 on January 26, 2027. September is the first full month on that grid, and it holds three majors: 155 on the 1st, 156 on the 15th, 157 on the 29th. That is the workload Ledru said would not require twice the speed, only twice the number of trains.

THE SEPTEMBER CLOCK

• September 1: Firefox 155.0 and ESR 153.2, 140.15, and 115.40, with advisory 2026-82.
• September 4: Firefox 155.0.1 notes, a point fix three days after the first two-week major.
• September 15: Firefox 156, the second two-week major, with ESR 153.3 listed on the same day.
• September 29: Firefox 157, the third major of the month, with ESR 153.4.

Most Release-channel users will not notice the new math. Automatic updates already install in the background. Anyone who wants a pause can still set Firefox to check and wait, under Settings, General, Firefox Updates. Organizations that need a frozen feature set still have ESR 153. The people who will feel 156 on September 15 are the ones already on the clock: release owners, locale teams, add-on authors, and the Gecko forks that followed Tails in saying the 14-day train is now theirs.

Ledru’s July note still stands. Mozilla will watch how the change works and adjust if needed. Firefox 155 got out the door on the new day, then needed 155.0.1 72 hours later. The next test is whether 156 can do the same job on 14 days of Nightly without turning that point-release habit into the process.