Log in to your AOL Mail account and select Settings at the top right of your browser. Select More Settings from the bottom of the dropdown.
On your left menu, select Writing Email.
Under the Signature heading, enable the toggle next to your email account.
Paste your copied HTML email signature into the signature field. You will see the Preview Signature window automatically populate with your email signature.
Close out of your settings and compose a new email. Your email signature will now automatically insert when composing an email from Aol Mail in Firefox.