I really love this add-on and thank you for making it.
However, I would really like it if your add-on honoured Thunderbirds settings for the use of the double-hypen -- signature separator.
I read your FAQ and the wikipedia article, but this is still inconsistent with Thunderbird itself. Also, the requirement of the -- separator as a standard is dying and even the wikipedia article you reference is under dispute on what is the standard for a separator.
This is most evident that even the coders for Mozilla no longer think that the -- is mandatory. In the Advanced Config Editor you can tell thunderbird to suppress the signature seperator and this works fine if you have signature set inside Thunderbirds account configuration. However, your add-on always adds the -- regardless of what setting thunderbird is configured.
Again, it would seem that if Mozilla is OK with signature separator suppression, then your add-on should conform to Thunderbird.
You can see the documented support for suppressing the separator here: