By default, Mozilla Thunderbird uses the reply string of “Joe Soap wrote:” . That is the string that appears at the top of your reply (if you’re using in line replies). You can change that, by editing your prefs.js.

1. Close Thunderbird
2. Open your prefs.js (on recent versions of Windows this file is located in c:\Documents and Settings\<your_username>\Application Data\Thunderbird\Profiles\<some_random_digits>\prefs.js)
3. If you don’t already have the following lines, add them at the end. Otherwise just change the existing ones to your satisfaction. Use the notes below to customise the string….

   user_pref("mailnews.reply_header_type", 3); 
   user_pref("mailnews.reply_header_authorwrote", "%s said the following");
   user_pref("mailnews.reply_header_ondate", "on %s");
   user_pref("mailnews.reply_header_separator", " ");
   user_pref("mailnews.reply_header_colon", ":"); 

The first line above (“mailnews.reply_header_type”) is a value of 0-3
0 = Don’t use any reply text (just the reply, no “Joe Soap wrote:”).
1 = The default “Joe Soap wrote:” format. Joe Soap is the friendly name of the original sender, or their email address if no friendly/display name was specified, or found in the address book.
2 = Takes the form of “On <date> <author> wrote:”, again <author> is the friendly/display name, if found, or just the email address of the original sender
3 = Custom string, which is defined by the following series of prefs.

The other preference lines above should be reasonably self explanatory, and should allow for most static type reply strings you’d need.