load auto fill? and auto post info when composing and e-mail
How do I load the Form auto fill function
How do I add a signature block that is automatically added when composing an e-mail?
Firefox doesn't do email, so it doesn't have a feature to autofill a signature.
If you are using Firefox to access a webmail portal, that feature would be provided by that portal's software. Usually part of HTML format email, or "rich text" email. See your webmail service provider's help files or support forum for help with that.