http://www.trafficisgold.com/ A Step by Step video guide on How to add an email Signature in Thunderbird. Thunderbird is a Mozilla email client and a free alternative to Microsoft Outlook 2007 and 2003. Step One: Create an HTML signature file: Step Two: Add signature file in Thunderbird Tools Account Setting Pick the identity you want to add a Signature to Check box that reads: Attach This signature. Check the box, then choose the file we created prior. Click OK Try composing an email and see if your new signature appears