I don't think you'll find the dash-dash-space in the signature itself. I think it will be added by Thunderbird at the point of use.

I couldn't work without that separator. I switch signatures, and the tools I use rely on that distinctive dash-dash-space sequence to determine which part of a message is signature. I think it's impolite to fill my correspondents' inboxes with multiple repeats of the ugly verbose signature that my employer requires me to use. So I use it once, at the start of a conversation, and thereafter I'll use a friendlier cut-down version.

However, there is a so-called "hidden setting" which lets you suppress the signature separator.

http://kb.mozillazine.org/Signatures_(Thunderbird)#Removing_the_signature_separator

Thank you, Zenos, for your prompt reply which worked perfectly.

However, I then realised that I was asking the wrong question, and by selecting "start my reply above the quote" and "place my signature below my reply" the separator disappeared.

I have therefore reversed the "suppress separator" setting.

However, one little quirk - when starting a new email, or replying, the signature appears exactly as required. However when forwarding, the signature appears with an extra line feed before it and after it.

Not very serious, but is there a tweak?

Thanks and regards

Willl