Mozilla Mailnews FAQ

What are structured words, and how do I disable it in Mozilla?

Structured words (also referred to as "plaintext tags") is a way of adding some basic HTML tags, while keeping your messages in plain text format. Mozilla knows how to display structured words, and displays them as such:

Text sent as:	It will appear as:
*bold*	*bold*
/italics/	/italics/
_underline_	_underline_

To disable structured words in Mozilla, add this line to your your user.js or prefs.js:
user_pref("mail.display_struct", false);

Or you can enter about:config in the location bar.
Search for the preference mail.display_struct.
Modify the value to false.