See: *http://kb.mozillazine.org/Preferences_not_saved *https://support.mozilla.org/kb/Preferences+are+not+saved ---- Do a malware check with some malware scanning programs on the Windows computer.<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 /><br /> *http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware *http://www.superantispyware.com/ - SuperAntispyware *http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner *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

Than you cor-el for your fast help. I'll do as you say and I'll post the results. For what I can infer there is not a way to get rid of this problem inside Firefox. Of course, I'll contact also the involved company, if possible, to complain about this uninvited intrusion.