The Yahoo toolbar is not a part of the default Firefox form Mozilla. Did you install one of the Yahoo-enhanced versions of Firefox that Yahoo make available? The Yahoo toolbar may also have been added when you installed some other software.

To uninstall the Yahoo toolbar, there is usually a pencil icon in the toolbar, click on it for the option to uninstall. If there is no pencil icon, use teh add-ons manager to uninstall the toolbar - https://support.mozilla.com/kb/Uninstalling+Firefox

Thank you. I must have unwittingly installed Yahoo's version and have now uninstalled it, but with a bit of effort. They didn't make it easy.

Thanks for your help.