It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder. Delete the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder. * Help > Troubleshooting Information > Profile Directory: Open Containing Folder * http://kb.mozillazine.org/Profile_folder_-_Firefox * http://kb.mozillazine.org/sessionstore.js If you see files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js then delete those as well.<br /> Deleting sessionstore.js will cause App Tabs and Tab Groups to get lost, so you will have to create them again (make a note). See: * http://kb.mozillazine.org/Session_Restore ----- Start Firefox in <u>[[Safe Mode]]</u> to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes). * Don't make any changes on the Safe mode start window. * https://support.mozilla.com/kb/Safe+Mode * https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes ---- Do a malware check with some malware scanning programs.<br /> You need to scan with all programs because each program detects different malware.<br /> Make sure that you update each program to get the latest version of their databases before doing a scan.<br /> * http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware * http://www.superantispyware.com/ - SuperAntispyware * http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page * http://www.safer-networking.org/en/index.html - Spybot Search & Destroy * http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free See also: * "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked