Go to the menu then go to the Add-ons then select Extensions and look for any unfamiliar or suspicious looking extensions. When you find any unfamiliar or suspicious looking extensions remove them. Do the same in your system's Control Panel. However,most malware will be 'hiding' somewhere , so its better to run malware scans. For more information : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no Run all of the listed malware scanners. They all work differently ( what one program doesn't pick up, the other might)

Well said.