how about forgetting searching the profile folder for files than probably do not exist. When Thunderbird is opened as you suggest the compose window uses the "default" snding account. This is the acount shown at the top of the list of accounts in the "folder" pane (unless you use the global inbox) in which case it should b listed first in the account setting list of accounts. Note that you may have set an identify on that account, so click the manage identities button to see what identities for the account are configured. Each identity can have it's own signature.