User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0 Build ID: 20150826023504 Steps to reproduce: In the version 38.2.0, I added approved: header within the compose window into a message then sent. This is a part of workflow supported by a server. Actual results: All headers are cleared out and cannot send mail anywhere. To make it work, I had to downgrade to 31.7.0. Expected results: When I add an arbitrary header, I can send mail without changing headers.

How do you add that header? Please specify the steps so we can reproduce it.

In about:config, I added the value "Approved" in mail.compose.other.header Then in a composer window, I select Approved: header from the drop-down menu which has To:, Cc:, Bcc:, and Reply-To: then type the value of a password required in the server-side. Then I click "Send" button to send mail.

I assume you also have a To: header so that the mail can be sent. I have tried it (both To: and Approved:) and yes, both headers are cleared but the message is "sent", e.g. stored in Outbox, if I click Send later. While sending this message I get an error in Error console: Error: TypeError: invalid 'in' operand addr Source File: resource:///modules/jsmime.jsm -> resource:///modules/jsmime/jsmime.js Line: 3068 That one looks relevant. The line is if ("email" in addr) For the Approved header, the "addr" variable contains a string (the value of the header) and is not an object. Jcranmer, is this something for you?

Certainly, I also entered To: and Cc: in the composer pane. Since the mail server received the mail, the To: header seems to be alive but no To: header appears in the message in Outbox.