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 comm-central, 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 comm-central.

Thunderbird Beta

• Release Timeline:

  • A new major version of Thunderbird Beta begins every 4 weeks.

  • 4 weeks after Daily starts for a new version, comm-central is merged into comm-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, comm-beta is merged into comm-release.

  • 1 week after this merge, the monthly Thunderbird Release is published.

• Releases: Thunderbird Release is released every 4 weeks, with point releases as needed every 2 weeks.

Thunderbird ESR (Extended Support Release)

• Release Timeline: The code from comm-release is merged into comm-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 comm-central prior to merging into comm-beta.

• During this time:

  • Risky code should not land in comm-central.

  • 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 136

Milestone	Date
Thunderbird 136.0a1 starts	Jan 6
Thunderbird 136.0a1 soft code freeze	Jan 27–Feb 3
Thunderbird 136.0a1 pre-merge review	Jan 30
Thunderbird merge 136.0a1 central → beta	Feb 3
Thunderbird 136.0b1	Feb 5
Thunderbird 136.0b2	Feb 12
Thunderbird 136.0b3	Feb 19
Thunderbird 136.0b4	Feb 24
Thunderbird 136.0b4 pre-merge review	Feb 24
Thunderbird merge 136.0b4 beta → release	Feb 25
Thunderbird 136.0	Mar 4
Thunderbird 136.0.1	Mar 18

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Thunderbird Milestones Calendar

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