For Firefox, open FF, click on "Tools", then on Add-ons, or Extensions. Find the Yahoo Toolbar listing and click on it, Select Uninstall, choose "yes/OK", when it asks: "Are you sure?".

For Internet Explorer, see the link below. (Uninstall it from the "Pencil" icon in the toolbar itself.):

http://help.yahoo.com/l/us/yahoo/toolbar/toolbar8/general/general-07.html

Hope this helps.

Grif