When Is the Next Firefox Release and Merge Day?

Updated

According to the current snapshot on WhatTrainIsItNow.com, Firefox 159 ships on October 27 (in 6 days), and the next merge day is October 22. Those two dates are the ones to plan around if you track Firefox trains: the merge day moves Nightly into the Beta slot, and the ship date is when the Beta build becomes the Release version. Because train dates can shift, treat these as the latest published values and re-check the site before you rely on them for a deadline.

What the site currently shows

The homepage presents the active trains as a set of version numbers, each tied to a channel:

Channel Version shown What it means
Nightly 160 The newest development train, not yet stabilized
Beta 159 The build being tested before release
Release 158 The current stable version users have
ESR 153 / 115 Extended Support Release lines, kept on a slower cadence

The page also states the two key dates directly: Firefox 159 ships on October 27 (in 6 days) and Next merge day: October 22. The "in 6 days" phrasing is relative to when the page was generated, so the countdown is a convenience, not a fixed fact — the absolute dates are the reliable part.

How release and merge day fit together

Firefox development runs on a "train" model: work moves through channels on a schedule rather than shipping whenever a feature is ready. The two dates on the site mark the handoffs in that pipeline.

  • Merge day (October 22) is when the next version is promoted into Beta. In practice, the Nightly train (160) becomes the new Beta, and a fresh Nightly cycle begins. This is the point where feature work stops landing freely and stabilization starts.
  • Ship date (October 27) is when the Beta train (159) is released as the new stable version, replacing 158 in the Release channel.

So the sequence is: merge on the 22nd, ship on the 27th. If you maintain add-ons, run a QA process, or write documentation, merge day is your signal that the API surface is settling, and the ship date is when users actually receive it.

Using the calendar for milestones

The site includes a Calendar section alongside the train overview, intended for following major milestones rather than just the current version numbers. Use it when you need dates beyond the immediate release — for example, to find when a future train is scheduled to merge or ship so you can plan testing windows or content updates.

A practical workflow:

  1. Check the homepage for the current channel-to-version mapping and the next two dates.
  2. Open the Calendar to confirm whether a later milestone you care about is already listed.
  3. Note the absolute dates (not the "in N days" countdown) in your own tracker.
  4. Re-check a few days before the merge day, since schedules can move.

Why you should re-check before relying on a date

The site itself frames these as dates that can change, and that is the right posture. Merge and ship dates are planned targets; a delayed merge or a held release will move them, and the countdown on the page will update accordingly. If a date matters to a deadline — a compatibility test, a support article, a training session — confirm it on the site close to the event rather than planning months out from a single reading.

For the broader picture of what each channel and version number represents, see the related explainers on what a Firefox train is and which version sits in each channel.