There is no place in Firefox to add a signature, it is just a web browser and has no email features. When accessing email via Firefox, any email features will be part of the email site, you need to check with the support for the email site that you are using to see how to add a signature.

You might want to look into the WiseStamp extension. You can add signatures to a plethora of websites. I still suggest that you contact the support of the email site that you want to insert a signature into and they can probably assist you without the need for an extension. If you have more questions about Wisestamp, please see http://www.wisestamp.com/usage-faq