Support for legacy WebExtensions was removed from Thunderbird Beta version 74, released in February 2020. Only modern WebExtensions are now compatible with Thunderbird 78.
There are two types of changes which are required to make your legacy extensions compatible with Thunderbird 78:
The legacy WebExtension must be converted to a modern WebExtension
All extensions need to be updated to reflect changes in Thunderbird core, like renamed/replaced API calls, removed support for some XUL elements and much more.
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