Patti, you need to click the start menu and click the arrow (right after the shut down button, but don't click shut down), then click log off. Your new user account will be one of those profiles. Firefox has nothing to do with user accounts, it is a program on each of the user accounts.
Also, A likely place that the trouble is coming from is within control panel. Uninstall everything that says yahoo in control panel - uninstall programs. Let me know if you want me to post a picture.
These problems are probably the same going on in this thread
(click to be linked to it) anyway, and it may help to have everything together so we know all of what is going on.