probably changed homepages to yahoo etc.
open firefox
right click on the top section that displays the buttons home / star favorites / - 100% + / etc / etc - not the actual buttons
select customise from the drop down list
look for a COG shaped circle called options ... drag to your bar
open options
general tab > show home page > change homepage address from yahoo to
https://www.google.com or whatever you want it to be.
or press restore defaults.
search tab > change from yahoo to google
now close that whole options tab and return to main browser screen
in address bar type about:addons
extensions tab > remove yahoo
plugins tab > remove yahoo
enjoy life without yahoo.