Firefox and Thunderbird Move to Two-Week Major Releases With Version 155

Firefox and Thunderbird now ship a new major version every two weeks. Firefox 155 arrived Sept. 1, two weeks earlier than its original Sept. 15 date, and Thunderbird 155 followed a few hours later, The Register reported.

Mozilla director of engineering Sylvestre Ledru announced the change on the dev-platform mailing list on July 9 and described it as an experiment, IT-Connect reported. He gave three goals. A finished fix that misses a release should wait about two weeks instead of a month. The schedule should be more predictable. And there should be fewer “uplifts,” the emergency path that copies a fix straight to Beta or Release and skips stabilization. “Work that is not ready should not be rushed, and features can still take the time they need to mature,” Ledru wrote.

The published schedule runs Firefox 156 on Sept. 15 and 157 on Sept. 29, then a release every other Tuesday through Firefox 164 on Jan. 26, 2027. Nothing yet commits Mozilla to the cadence after that. All three major browsers are converging on it. Google announced two-week Chrome releases in March, starting with Chrome 153 on Sept. 8, and Microsoft’s Edge 152 moved to the same pace on Aug. 27.

What’s in 155

Firefox 155 focuses on AI. The Smart Window feature reaches more people in more regions, and LLM-assisted tab-group naming has been improved. The release also adds a blocked-tracker count in the address bar (on a progressive rollout), lets users reorder containers, supports Happy Eyeballs v3 for faster connections and adds Marathi and Urdu to local translation. Users who don’t want the AI features can turn them off with the “AI kill switch,” available since Firefox 148.

Thunderbird 155 adds an enterprise policy that disables QR-code export of settings to Android, which The Register noted could be a corporate security risk, plus a policy to turn off experimental features. It also improves handling of S/MIME-encrypted attachments, adds finer control over OAuth sign-in flows, and fixes about 30 bugs along with several vulnerabilities.

ESR for those who want off

Admins who don’t want updates every two weeks should move to the Extended Support Release rather than turn off automatic updates. Version 153 is the new ESR for both apps, and it’s the first long-term Firefox release that includes the AI kill switch. ESR 140 reaches end of life on Sept. 15. ESR patch releases follow the public calendar, so ESR security fixes will now also arrive every two weeks. Mozilla’s localization team said the gap between string freeze and release shrinks from three weeks to two.