"other header" pref ("mail.compose.other.header")
The format for this a comma delimited list of mail headers, like "Approved,X-No-Archive"
An example for your prefs.js would be:
This will cause "Approved" to show up in the compose window address picker, under To:, Cc:, Bcc:, Newsgroup:, etc.
When that mail gets sent,
will be in the message in the headers.
This is a useful trick for newsgroup moderators who want moderate from mozilla.