So I tried searching for the Yahoo Mail Preferences and I couldn't find anything other than the settings gear icon. I tried clearing all history from my Firefox Privacy options and they still show up. I don't know what else to do or find, can you please guide me?

I'm a complete novice when it comes to PC so please bear with me, so recently I've just discovered that even though I remove an email message and delete a contact from my contact list permanently, the old email address from that specific person still appears whenever I type it on the "search" mail option. And they show even more old addresses on the smart views important option. How can I remove them all, or is there even a way? I would very much appreciate your help. One guy answered me something like this: "It is called "autofill" - you can disable it in the browser's Preferences menu." And I didn't understand anything about it. So I tried searching for the Yahoo Mail Preferences and I couldn't find anything other than the settings gear icon. I tried clearing all history from my Firefox Privacy options and they still show up. I don't know what else to do or find, can you please guide me?