when I start a new tab yahoo search comes and I don't want that. how do I get a different web site to open?

You need to figure out what you have installed which changed that action to Yahoo from a blank tab, which is the default action in Firefox; and then get rid of that add-on. Then you can can install an add-on which allows you to select what you want in the new tab. https://addons.mozilla.org/en-US/firefox/addon/newtaburl/