The dash dash space return sequence is an informal standard for indicating a signature. It originated in newsgroup postings but Thunderbird has preserved it for use in email.

So what Thunderbird is doing is deliberate, but will happen only in those cases where your correspondents use this convention. It seems to me to be very sensible; when you reply, your correspondent doesn't need his own signature reflected back to him. But none of my Microsoft-using colleagues see anything amiss with this repetition and inevitably, all correspondence with Outlook users is burdened with multiple instances of signatures, complete with logos etc.

The vast majority of my email correspondence follows patterns set by Microsoft, where it's all top-posted with horizontal rules indicating the breaks between previous messages in a chain. I envy you having mature correspondents who do it the better way.

There is a strong tie-in with top-posting here too. If you do as Thunderbird does, your signature is added at the very bottom, (regardless of whether you top-posted, bottom-posted or interleaved) where it can be automatically trimmed off. But Microsoft have trained users to top-post, usually with full signature quoted and re-inserted at the top each time. This makes any automated clean-up impossible.

If you really want to retain the original signature you probably need to use "copy-and-paste-as-quotation" on the original message then remove the dash dash space line from the pasted text. Or try setting the mail.strip_sig_on_reply preference to false in the config editor.

For your own messages, you can set Thunderbird not to use the dash dash space delimiter. This is quite a common request from users who dislike seeing the '-- ' and presumably have no interest in eliminating unnecessary content. Look for the mail.identity.default.suppress_signature_separator preference in the config editor and set it to true.

The '-- ' is essential if you use an add-on such as Signature Switch, since it needs this to identify where the signature starts.

As an aside, by default Thunderbird shows signatures in a pale font colour, either by using grey or by applying partial transparency. This also annoys users who see it as distorting their carefully crafted signatures. This feature may be behind the different colours you saw when selecting copying. the shading effect can be turned off; I use some css code to do this.

When I started to read your reply, Zenos, I recalled this feature. I've seen it before during my 30 years long time in the computer business. Probably these 30 years made I forgotten it ;). Now I've tried to implement it in my Thunderbird e-mail client. I defined signature in my hotmail account setup in Thunderbird. See the uploaded image. I wrote an e-mail message and I sent it from my hotmail account to my gmail account using Thunderbird. Then I sent a reply from my gmail account using Thunderbird. What I've got is displayed in attached image. Why my signature is displayed in the reply? Have I've done anything wrong? I won't be monitoring the Mozilla Support in the next 3 weeks.