Update. Since I posted this I have found the add-on again. In fact it is now available from the Mozilla Add-Ons site. There is a new home page for it too.
I posted some time ago about Firefox add-ons. One I mentioned was Blank Canvas Gmail Signatures which allowed you to have different signatures for each Gmail account. Unfortunately it is no longer available on the Mozilla site. There is no mention of it on the Blank Canvas site either.
This is a great loss as it is the only Gmail signature add-on that supported multiple accounts. There are other add-ons for signatures but none of them have this feature. The xpi file can still be found on archive sites, a search should bring up a source for you.
Hopefully one of the other add-ons will add this feature or even better Gmail itself will provide it.