> For the complete documentation index, see [llms.txt](https://developer.thunderbird.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.thunderbird.net/releases/cadence.md).

# Release Cadence

## Thunderbird Release Train Model

Thunderbird follows a release train model to ensure timely and predictable releases. This approach allows for regular feature rollouts, stability improvements, and bug fixes.

## General Release Cadence

The general release cadence consists of 4-week cycles, with each branch corresponding to a specific release channel. Each branch, starting from main, is based on the previous branch.

![](/files/mHJR9gqvhjXL1Tvjc3iw)

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## **Release Channels**

### **Thunderbird Daily**

* **Release Timeline**: A new major version of Thunderbird Daily begins every 4 weeks.
* **Releases**: Thunderbird Daily is automatically released on a daily basis from main.

### **Thunderbird Beta**

* **Release Timeline**:
  * A new major version of Thunderbird Beta begins every 4 weeks.
  * 4 weeks after Daily starts for a new version, main is merged into beta.
* **Releases**: Thunderbird Beta for this version is released weekly for the next 3 weeks.
  * **Beta 1**: Includes the merged code.
  * **Beta 2 and above**: Include any new uplifts.

### **Thunderbird Release**

* **Release Timeline**:
  * A new major version of Thunderbird Release begins every 4 weeks.
  * 3 weeks after Beta starts for a new version, beta is merged into release.
  * 1 week after this merge, the monthly Thunderbird Release is published.
* **Releases**: Thunderbird Release is released every 4 weeks, with a point release, as needed, 2 weeks after release.

### **Thunderbird ESR (Extended Support Release)**

* **Release Timeline**: The code from the release branch is merged into esr\<version> once a year.
* **Releases**: Thunderbird ESR is released every year, typically in July, with point releases as needed every 2 weeks.

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## **Stabilization Milestones**

### **Daily Soft Code Freeze**

* A 1-week soft code freeze occurs for main prior to merging into beta.
* During this time:
  * Risky code should not land in main.
  * Features controlled by a feature flag that were disabled in Daily should not be enabled.

### **Pre-Merge Reviews**

* Prior to both merges, reviews are conducted for all changes included in the merge.
* Code may be backed out if:
  * It is deemed too risky.
  * It introduces new crashes or high-severity bugs.
  * It reduces the overall quality of Thunderbird.
  * It introduces a bug and a follow-up fix cannot be provided in a reasonable timeframe.

***

## **Sample Release Timeline for Thunderbird 154**

| Milestone                                | Date          |
| ---------------------------------------- | ------------- |
| Thunderbird 154.0a1 starts               | Jun 22        |
| Thunderbird 154.0a1 soft code freeze     | Jul 13–Jul 20 |
| Thunderbird 154.0a1 pre-merge review     | Jul 16        |
| Thunderbird merge 154.0a1 main → beta    | Jul 20        |
| Thunderbird 154.0b1                      | Jul 22        |
| Thunderbird 154.0b2                      | Jul 29        |
| Thunderbird 154.0b3                      | Aug 5         |
| Thunderbird 154.0b4 pre-merge review     | Aug 6         |
| Thunderbird 154.0b4                      | Aug 12        |
| Thunderbird merge 154.0b4 beta → release | Aug 12        |
| Thunderbird 154.0                        | Aug 18        |
| Thunderbird 154.0.1                      | Sep 1         |

***

## **Thunderbird Milestones Calendar**

You can view Thunderbird release milestones on the [Thunderbird Release Calendar](https://calendar.google.com/calendar/u/0/embed?src=c_f7b7f2cea6f65593ef05afaf2abfcfb48f87e25794468cd4a19d16495d17b6d1@group.calendar.google.com).