Seriously I have no idea, but I would assume that the signature switch add-on may well assume the delimiter is the standards compliant -- Given Thunderbird uses the -- to determine where the signature is when you change identities in a compose window I would assume the signature switch add does the same. Really you will need to ask the add-on author. It might be worth noting that the signature switch add-on is only supposed to work up to Thunderbird V38 and without an update is unlikely to work at all in the upcoming Thunderbird 60.