The spec says
The nickname chosen by the sender of the message, which can be different for each message in a conversation.
Besides sender-nickname, check other changes in the header for removals/additions (protocol-token/message-token/...) and also bind MessageTypes, falling back to GetMessageTypes if the property does not exist.
These keys added in spec 0.19.8 are quite important, yet we haven't added friendly accessors for any of them.
Actually we've had this since 0.7.0, except for MessageTypes which now has a separate bug 41356.
*** This bug has been marked as a duplicate of bug 41356 ***