Icedove: Cleanly reply to a message from a web archive

When you reply to a message, Icedove (Thunderbird) adds the <message-id> of the replied to message in your new mail's header field "In-Reply-To".

If you want to answer to a post in a mailinglist that you read in a web archive of the mailinglist, but hadn't been subscribed to it previously (so you don't have the replied-to message in your mailbox) you can do this manually (if the web archive lists the message id).
See http://kb.mozillazine.org/Custom_headers.

In Icedove: Preferences - Advanced - Config Editor
Select "mail.compose.other.header" and add
"In-Reply-To,References,X-Debbugs-Cc".

Restart Icedove.

Now you can add an In-Reply-To (like adding another To/CC) and insert the <message-id> of the post that you want to reply to.

The example also adds fields for References (the whole tree that you want to reply to) and X-Debbugs-Cc (needed when you submit a Debian bug and want to notify someone immediately).

EDIT 2015-08-29: How to add several fields.