Great idea and much needed.  I have the addon installed for several days now and it works for the first few times I send or reply to an email but after awhile it just stops working. I can hit compose or reply and select the account I want to reply with and the default box for the signature is missing and no signature is inserted.

I am a heavy Gmail user and have it open all day long.  If I close my browser and go back to Gmail the signature works just fine.  But then again after awhile it stops working.

Thank