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
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
So, I use chrome so i dont really know about firefox but in chrome you can open a new page go to where u type in something in the adress bar, right click and click on edit search engines and remove yahoo. If I helped please give a upvote! -Carter