In Firefox, go to Help - Restart with Add-ons Disabled. In that "Firefox Safe Mode" display that opens, place checks next to the following, then click "Make changes and restart".
Reset toolbars and controls
Reset all user preferences to Firefox defaults
Restore default search engines
You can change those later to whatever you prefer, but for now, too many search hijackers have altered things there.
Each of the later versions vary in these procedures, but whatever the Safe Mode option allows to be removed, it should be right.
That likely won't match whatever version of Firefox you have, since it seems to change with every version. But should be sufficient to walk you through resetting Firefox.