This message display scripts API first appeared in Thunderbird 82 and was backported to Thunderbird 78.4. Functionally it is the same as the content scripts API except that it works on the document of email messages being displayed. See the MDN documentation for a more in-depth explanation and Changes in Thunderbird 78.4.0 for examples.
Registering a message display script through
manifest.json is not possible at this point.
The permission messagesModify is required to use
Register a message display script programmatically
An object that represents a message display script registered programmatically
unregister()Unregister a message display script registered programmatically
The following types are not defined by this API, but by the underlying Mozilla WebExtension code base. They are included here, because there is no other public documentation available.
Specify code, either by pointing to a file or by providing the code directly. Only one of the two is allowed.