The -- is the signature separator, a standard element to prevent recipients from quoting the signature when they reply to your message.

http://kb.mozillazine.org/Signatures_-_Thunderbird#Removing_the_signature_separator

Thanks. First I've heard of that. I guess I need to consider the ramifications of turning that off.

As a tangent, maybe needing a new thread, I find attempting to use a file for "rich text" or other formats causes the signature to seem "garbled" displaying what seem to be file and formatting information.

Is that a different issue?

That's a different issue. If you make an html signature file with anything besides the TB message composer, you're more likely to encounter formatting problems.

https://support.mozilla.org/en-US/kb/signatures#w_html-signatures