Reply above quoted message
From MozillaZine Knowledge Base
When replying to messages, you may want your reply text to appear above (or below) the quoted text by default. This can be set differently for each account. Go to Tools | Account Settings, and then for the account you want to change select Composition & Addressing. Using the drop-down list on the right, choose the reply position you want to use and click OK.
Some information on mail reply and quoting format can be found here: http://mailformat.dan.info/quoting/