hello thomas, if you haven't created it yourself in the first place or aren't sure what has created it, then the user.js file shouldn't be there at all. you can go ahead and delete the file - it won't affect your stored bookmarks or other personal data...

Thanks Philip for your quick answer!

Do a malware check with some malware scanning programs on the Windows computer.<br> Please scan with all programs because each program detects different malware.<br> 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 *SuperAntispyware:<br>http://www.superantispyware.com/ *Microsoft Safety Scanner:<br>http://www.microsoft.com/security/scanner/en-us/default.aspx *Windows Defender: Home Page:<br>http://www.microsoft.com/windows/products/winfamily/defender/default.mspx *Spybot Search & Destroy:<br>http://www.safer-networking.org/en/index.html *AdwCleaner:<br>http://www.bleepingcomputer.com/download/adwcleaner/<br>http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml *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

Thanks so much for your great answer. Sorry for not replying sooner.