Comment 34 for bug 357864

The UI for custom headers is already in the Message Filters window, under
"Customize..." in the pulldown list of headers.

I suggest adding this UI also to the Compose window. It already uses pulldown
headers, but they are hardcoded to a predetermined set of 6: "To", "Cc", "Bcc",
"Reply-To", "Newsgroup", and "Followup-To". It seems straightforward to add
"Customize..." below this hardcoded list.

When this is done, and "From" is entered as a custom header, it would override
the noneditable "From" address displayed above the header list. The user could
enter a custom address, and the GUI could perhaps keep it in sync by changing
the displayed "From" address above to match this header. This would be a
special case, though, and I'm not sure if this is the right thing to do. Also,
multiple "From" headers are invalid, and would need to be blocked by the GUI.

Or perhaps it's easiest just to make the From field, as displayed above the
header list, editable? A checkbox, hidden away in Advanced preferences so
novice users won't check it by mistake, would be fine. If the From field can be
made editable in this way, it could then be blocked from inclusion in the custom
headers above, avoiding the complications described above.

So, the combination of these two features would work very well with each other:
* Advanced option to make the displayed "From" field editable.
* "Customize..." in the pulldown header list when composing a message, to allow
all custom headers to be added, except From (which is a special case).

I really hope this makes it into Mozilla, as it's the one reason I still go back
to Eudora sometimes when sending mail!