In a mail message, Thunderbird prefixes the signature with the line "-- " (two dashes and a space). I can't find an option to prevent it from inserting this line. When you reply to a message, Thunderbird apparently takes the "-- " as a marker to stop copying the original message. To re-produce the bug: 1. start thunderbird 2. set signature 3. compose a new message as this (there is a space after dashes): Hello -- Hello 2 4. send it to yourself 5. reply this e-mail to yourself expected result: "Hello 2" should be part of message actual result: "Hello 2" is missing

Nope, there isn't any option.