get rid of all the adds, default search machine on the current version of firefox (31.0)

I had to clean up my pc because of an adware problem when I tried downloading Chrome. I opened the current version of firefox and a display appeared "Congratulations you have the latest version of Firefox. Download a fresh version." I downloaded the fresh version. Since that download, I constantly receive unwanted adds and when I open Firefox, a default search machine "iStart123" is loaded even though I have my e-mail browser set up to be displayed when Firefox is opened. Now, how do I get rid of this "fresh version" of Firefox and how do I get rid of this "iStart123" and all those unwanted adds? Thanks for a quick reply. This "fresh version" of Firefox is about to drive me up a wall. Chuck