Required steps to update add-ons for Thunderbird 91.
Support for legacy extensions was removed from Thunderbird Beta version 74, released in February 2020. Since Thunderbird 78 only modern MailExtensions are supported. This section only covers the required update steps for add-ons which are already compatible with Thunderbird 78 and need to be made compatible with Thunderbird 91.
Changes in WebExtension APIs
Our WebExtension APIs are meant to be stable, but we are adding new features and since this is a very young technology this might also require backward incompatible changes. All WebExtension API changes are listed in our API documentation: