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Thunderbird 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.

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 point releases as needed every 2 weeks.

Thunderbird ESR (Extended Support Release)

  • Release Timeline: The code from comm-release 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.

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 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 main → 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

Thunderbird Milestones Calendar

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