As part of its plan to modernize Thunderbird, Mozilla is phasing out old legacy add-ons that are built on the XUL and XPCOM APIs. While engineers have already started work on adding support for WebExtensions, there is no word on when this feature will land in a stable release, nor when Thunderbird will actually stop supporting legacy add-ons.



Mozilla engineers are currently providing information to add-on developers on how to migrate their add-ons to the new WebExtensions API, similar to how they’ve mentored Firefox add-on devs before the release of Firefox 57 (the first Firefox version that dropped legacy add-ons).

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Source: [H]ardOCP – Thunderbird Will Phase Out Legacy Add-Ons for WebExtensions