You can check for recently installed suspicious or unknown extensions. *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems Do a malware check with several malware scanning programs on the Windows computer. *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Please scan with all programs because each program detects different malware. All these programs have free versions. Make sure you update each program to get the latest version of their databases before doing a scan. *Malwarebytes' Anti-Malware:<br>http://www.malwarebytes.org/mbam.php *AdwCleaner:<br>http://www.bleepingcomputer.com/download/adwcleaner/<br>http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml *SuperAntispyware:<br>http://www.superantispyware.com/ *Microsoft Safety Scanner:<br>https://www.microsoft.com/security/scanner/en-us/default.aspx *Windows Defender:<br>https://windows.microsoft.com/en-us/windows/using-defender *Spybot Search & Destroy:<br>http://www.safer-networking.org/en/index.html *Kasperky Free Security Scan:<br>https://www.kaspersky.com/security-scan You can also do a check for a rootkit infection with TDSSKiller. *Anti-rootkit utility TDSSKiller:<br>https://support.kaspersky.com/viruses/disinfection/5350 <br>https://support.kaspersky.com/viruses/utility See also: *"Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked