Can someone please help me cancel my subscription to Fire Fox

You can do it on https://accounts.firefox.com/ page.

Cancel my subscription to the Fire Fox Web Browser I don't want it any longer

The Firefox web browsers for desktop and mobile versions for Android and iOS/iPadOS do not have any paid for subscription options.

Perhaps you are referring to one of Mozilla subscription based products as in Mozilla VPN, MDN Plus and the Plus or Premium versions of Firefox Relay, Pocket, Mozilla Monitor.