Parece haver um motivo específico pelo qual o recurso 'Inserir assinatura aqui' não está disponível no Thunderbird.
Esta entrada de FAQ do complemento do Thunderbird Switch de assinatura (sugerido por Horatio) realmente me forneceu algumas dicas sobre porque o Thunderbird provavelmente não tem esse recurso.
FAQ Question: My signature doesn't get removed when it is placed before quoted text. Is that a bug?
No it isn't. ;-) If your Thunderbird is set to place the signature below your reply (above the quote) the removal won't work.That's because Thunderbird then doesn't insert the standard-delimiter ("-- \n") before the signature (for a good reason!). And that's what "Signature Switch" is looking for.
Some people might try to bypass that problem by manually adding the sig-delimiter to their signature file. But that will cause another problem. The "-- \n" always marks the very end of a mail-message. "Signature Switch" assumes that any text placed below the signature-delimiter (including any quoted lines) is part of the signature. ... and therefore removes everything below the delimiter.
Besides... this option isn't recommended by Thunderbird itself! So please put your signature below your quotings. (check the Wikipedia-excerpt below)
aqui está o trecho:
From Wikipedia: " ... The formatting of the sig block is prescribed somewhat more firmly: it should be displayed as plain text in a fixed-width font (no HTML, images, or other rich text), and must be delimited from the body of the message by a single line consisting of exactly two hyphens, followed by a space, followed by the end of line (i.e., "-- \n"). ... "
e finalmente:
FAQ Question: I want to place my signature at any desired position within the message. Could you please implement cursor-point-insertion to "Signature Switch"?
Definitely no. Sorry. (And please don't ask me to reconsider.) Due to the reasons explained for the previous question I can't offer you this. Besides... something like that wouldn't have anything to do with "signatures" anymore.
If you're looking for an easy way to paste predefined text please check out other extensions especially made for that purpose.
Ainda assim, tudo bem e correto do ponto de vista técnico, mas em termos de usabilidade eu gostaria de inserir minha assinatura onde quiser.