You can check for problems caused by a possible corrupted user agent. See: *https://support.mozilla.org/kb/Finding+your+Firefox+version *https://support.mozilla.org/kb/websites-say-firefox-outdated-or-incompatible You can check how websites see you on the browserspy.dk website. *http://browserspy.dk/useragent.php Such an issue can also be caused by security software. ---- Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps. *http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/ ----- Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions. ---- Make sure that 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>http://www.microsoft.com/security/scanner/en-us/default.aspx *Windows Defender:<br>http://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>http://www.kaspersky.com/security-scan You can also do a check for a rootkit infection with TDSSKiller. *Anti-rootkit utility TDSSKiller:<br>http://support.kaspersky.com/5350?el=88446 See also: *"Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked *https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware