ThunderWiz

A signature randomizer for Thunderbird and other email clients:
This app empowers you to use dynamic signatures in email clients supporting signature files.

PLEASE NOTE: Apple Mail does not currently support external signatures.

This is how it works:

Simply select the signature file in your email client and enjoy dynamic signatures in your emails changing periodically according to a schedule you choose!
Once configured the signature will change even if the app is not currently running or your Mac is rebooted until the randomizer feature is turned off again in the app's settings.