Firefox is just a web browser and has no built-in signatures. Signatures are an email feature, and any email accessed through Firefox will be a webmail based email service, with the signature being part of the email service. You will need to check with the support for the email service that you are using for how to change the signature. If your question is about Thunderbird, for Thunderbird support you can ask in one of these forums: * http://forums.mozillazine.org/viewforum.php?f=39 * http://getsatisfaction.com/mozilla_messaging