hello annarees, since this can sometimes be caused by malware active on a system, please run a scan with different security tools like the [http://www.malwarebytes.org/products/malwarebytes_free free version of malwarebytes], [http://www.bleepingcomputer.com/download/adwcleaner/ adwcleaner] & [http://www.microsoft.com/security/scanner/default.aspx microsoft safety scanner]. [[Troubleshoot Firefox issues caused by malware]]

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test. *http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/