Just FYI this is help user forum not a support ticket request forum. AFAIK if you deleted the browser data and cookies they are gone. And lost logins isn't something the Forum can help with here. That is to contact Gmail support for lost accounts.

Hi, did you use the "Clear Data" button on the Settings page to clear the cookies and site data?

What was stored in your cookies and site data would have only have been a session token that lets Firefox tell the site that you already signed in. Sites do not store your actual password in a cookie.

I'm sure you know your GMail address to sign in to your Google account. Did you give Google a secondary email address or mobile number you can use to reset your Google account password? If not, or if that doesn't work, you may need to get onto Google's forums and ask them for advice.