Would you like to control where your reply appears (above or below the quoted text) in Thunderbird?. Here's how...
First choose Thunderbird -> Preferences -> Advanced -> Configuration Editor. In the list that appears, select mail.identity.default.reply_on_top. Set the value to 0 to have your reply at the bottom. Set the value to 1 to have the reply on top. You can change the default behaviour on a per-identity basis by adding a new line. For example:
mail.identity.id1.reply_on_top
Set the Status to User, Type to Integer, and add the preferred value, either 0 or 1.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006051605271325