This message display scripts API first appeared in Thunderbird 78. 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 Thunderbird 82 Beta for examples.
Registering a message display script in the manifest.json file is not possible at this point.
Read and modify your email messages as they are displayed to you
Transfer sensitive user data (if access has been granted) to a remote server for further processing
The permission messagesModify is required to use
Register a message display script programmatically. Note: Registered scripts will only be applied to newly opened messages. To apply the script to already open messages, manually inject your script by calling executeScript([tabId], details) for each of the open messageDisplay tabs.
An object that represents a message display script registered programmatically
unregister()Unregister a message display script registered programmatically
Details of 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.