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.

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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.

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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

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Thunderbird Milestones Calendar

You can view Thunderbird release milestones on the Thunderbird Release Calendar.