Hi,

Firefox and other browsers are just front ends that display email pages like any other web page. The actual emails, the settings, storage etc. are all stored and coming from the email provider's end and Firefox just formats and displays them nicely. You may have to go into the settings of the email account to see if the provider has an option to include images as signatures. If the provider hasn't setup an option to insert images as signatures, then it's may not be possible.