I use Thunderbird itself to create signatures. # Start a new message. # Delete any pre-existing content. # Create your signature as if it were an email message, using the tools in the compostion window to decorate your text, insert links and images. If you add images, be sure to go to the image's properties and set it to be attached to the message. # Save the message as html. Now you can enter the html file's name into account settings to use it for your signature. Or you can open your html file and copy its contents into the signature text box in account settings. Be sure to tick the "use html" checkbox. I generally make local copies of images used in signatures and store them alongside the html file, so I have the complete package to hand. If you need multiple signatures then the Signature Switcher add-on is useful. It gives you a toolbar button to select which html signature file to use.