When I hit reply to an email, the recipient's signature i removed if it is preceded by "--". Is there a way to stop this in Thunderbird? Many thanks!
This behavior is actually by design. The "-- " (dash-dash-space line) is the "signature cut line" and is placed by conforming mail clients (e.g., Thunderbird, Gmail, etc.) for precisely the reason that they can be trimmed automatically from responses.
Some background: http://en.wikipedia.org/wiki/Signature_block
I believe they originally came out of Usenet circles, and the idea was that if someone had posted to a list, the signature was already recorded and seen by subscribers. Since bandwidth was at a premium (is still, worldwide), cutting it would not needlessly waste time, space, and money.
As far as actively including it when replying, I don't know of a way. Either trawl the Thunderbird extensions, or hire someone to build one for you.
Thunderbird has a hidden option to do this. Go to Tools -> Options -> Advanced -> General -> Config Editor… Filter on mail.strip_sig_on_reply and toggle the preference to false.