I really like Thunderbird but I feel the Signature Function in Thunderbird is not well implemented. For example, the default option of editing signature text is very basic and the result looks ugly and unprofessional (see Picture1). HTML signature is also not user friendly for non-technical users. For some weird reasons, the signature is always located at the bottom of all emails contents instead of the bottom of the email the user is currently writing. (see Picture2) The signature completely not included in a email when forwarding a email (see Picture3).
I really love Thunderbird but I hope its signature function could be better implemented.