How To
Within Firefox

1. Open a new Tab, fill "about:config" in the address bar (without quotes) and press Enter key or click on the arrow
2. Fill "simplemail" in the filter area to get all related parameters
3. Find "extensions.simplemail.reply" and modify/paste a string to meet your need
4. Verification:
   go back to main window of Simple Mail, select a message and click on Reply button to see the result

Example
The following HTML string will get you a 2-line reply header including subject, GMT timestamp, your locale timestamp and the sender:
"
<span style='color: black; font-style:normal; font-size:12pt;'>{ <font color=green> {subject}</font> }</span><br><span style='font-size:10pt;'>{date-GMT}, </span><span style='color:black; font-style:normal; font-size: 12pt;'>{date-locale}, {author}:</span>
"
And it will look like:
{ Re: Another thoughts }
2008-05-27 12:40:01 GMT, 2008年5月27日 下午 08:40:01, <telega@gmail.com>:

Options
Here are the options available since version 2.66 (05-27-2008 09:34:41) to custom the reply header:

• {author}: email address of the sender.
• {date}: compact local date string, such as "27/05/08 20:40:01".
• {date-locale}: date string in your locale language, such as "2008年5月27日 下午 08:40:01".
• {date-GMT}: compact GMT date string, such as "2008-05-27 12:40:01 GMT".
• {subject}: subject of your message.

Use HTML tags to beautify your reply header.