My own add-on Mail Merge has supported custom headers for years. Custom headers can be defined by the user via the preference "mail.compose.other.header" and then used in the compose window.

However, at the moment it is not possible to access custom headers via ComposeDetails. There should be a new property "headers", which contains an array of the custom headers of the message.

This new property "headers" should be supported in getComposeDetails(), setComposeDetails(), beginNew(), beginReply(), beginForward() the new send function discussed in Bug 1545930.

This would also help the add-on Send Later as mentioned in Bug 1631580:

Set headers via setComposeDetails
The ComposeDetails should support a headers object to set headers. I guess we have to work with blacklists, as some headers probably should not be manipulated. There is a similar request here but it needs shaping.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/5790f457bfa6
Add support for custom headers to ComposeDetails. r=mkmelin