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 82 for examples.
Registering a message display script through
manifest.json is not possible at this point.
- messagesModify “Read and modify your email messages as they are displayed to you”
messagesModify is required to use
An object that represents a message display script registered programmatically
unregister()Unregister a message display script registered programmatically