There are a few different ways Yahoo might open the door for you automatically:

(1) Persistent cookie. By persistent, I mean that it remains valid after you exit Firefox and start it up again. To avoid having a persistent cookie saved, you can either:

• Log out of Yahoo before shutting down Firefox -- this is the most sure-fire way to prevent someone from sneaking into Yahoo because you had been logged in before

• Change the cookie permission for yahoo.com so that it can only save a session cookie. While on Yahoo, click the padlock in the address bar, click More Information, then at the top click Permissions. This will present a long list of settings for the site. In the "Set Cookies" session, uncheck the Use Default box and choose Allow for Session.

(2) Password Manager filling in the login. Firefox has a built-in password manager, and/or you might have an add-on password manager (such as LastPass or Dashlane). You can remove the saved login from the password manager, and not save it again.

For the built-in password manager, you can remove logins here:

"3-bar" menu button (or Tools menu) > Options > Security > "Saved Passwords"

For add-on password managers, it would vary depending on which one you have.

Does that help?

One other thought: if you have a Yahoo toolbar or other add-on, it might be storing your login so that all the features of the toolbar work.