You are refering to the signature delimiter. '''what does the delimiter do?''' http://www.ehow.com/info_12071771_email-signature-delimiter.html If the signature is placed directly after the reply and on top of the quote, it lacks the "-- " signature separator. That separator is used by Thunderbird to strip off the old signature when replying or changing identities. The signature becomes part of the message and can no longer be automatically removed (since it can't be identified using the seperator), causing a stacked signature ''' To switch it off for all identities of your accounts:''' Tools > Options > Advanced> General tab or menu icon > Options > Options > Advanced> General tab * click on 'Config editor' button it will tell you to be careful :) * in top search type: separator * look for this line: mail.identity.default.suppress_signature_separator; false * Double click to toggle 'false' to 'true' or right click on line and select 'Toggle' * Close config editor - top right X * click on OK to save changes in Options.