Form data is not cookies AFAIK.

Control whether Firefox automatically fills in forms

Note that a detail like a website remembering you (log you in automatically) is stored in a cookie.

I would advice not to use this for a banking website, but you can use this for a forum or other website you visit frequently that do not deal with financial transactions.

You can use these steps to make a website recognize and remember you:

• You can create a cookie "Allow" exception to keep specific cookies, especially in case of secure websites and when cookies expire when Firefox is closed.
• Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: Exceptions

Let the cookies expire when Firefox is closed to make them session cookies instead of using "Clear history when Firefox closes" to clear the cookies.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: Keep until: "I close Firefox"

You can create a cookie "Allow" exception to keep specific cookies.

You need to keep the Site Preferences.

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data