The mailnews backend component responsible for providing mail headers to the XUL/JS frontend (/mailnews/mime/emitters/src/nsMimeHtmlEmitter.cpp) doesn't provide all the headers in a message. It only provides a hardcoded list of common headers. This makes it difficult to customize the frontend to display custom headers. The code in question has this comment: optimization: if we aren't in view all header view mode, we only show a small set of the total # of headers. don't waste time sending those out to the UI since the UI is going to ignore them anyway. How much difference does this optimization make? The backend should either reflect all the message headers. If it actually impacts performance, it could take a list of headers from the frontend to concentrate all the logic regarding which headers should be displayed in the frontend.
*** This bug has been marked as a duplicate of 61523 ***