Such settings are stored in a cookie on your computer.

• Create an Allow cookie exception to keep such a cookie, especially for secure websites and if cookies expire when Firefox is closed.
• Tools > Options > Privacy > Cookies: Exceptions

In case you are using "Clear history when Firefox closes":

• do not clear the Cookies
• do not clear the Site Preferences

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

Clearing cookies will remove all specified (selected) cookies including cookies with an allow exception.

Placing an exception for Google Maps rolled back to Classic will not work, as it comes out as just google.com on the exceptions list. You have to make the roll back from within Google Maps, and I finally managed to do that. Have no idea why it was so hard to do, as the roll back worked first time in IE and Chrome. Google must have placed a cookie for this but I don't know which one it is. Thanks for the advice.