This could be an issue caused by AdAware or Web Companion from Lavasoft. You can look for a file named dsengine.js in these locations. You should only find channel-prefs.js in the "defaults\pref" location. Any file found here apart from channel-prefs.js is suspicious. You can check the content of the file in a text editor (use open with and do not double-click the file). C:\Program Files\Mozilla Firefox\defaults\pref\ C:\Program Files (x86)\Mozilla Firefox\defaults\pref\ You can look for a file named dsengine.cfg in the main Firefox program folder. C:\Program Files\Mozilla Firefox\ C:\Program Files (x86)\Mozilla Firefox\ Delete the dsengine.js and dsengine.cfg files when present. Search for Programs and Features, delete programs that you do not know San with https://www.malwarebytes.com/ & https://www.adlice.com/download/roguekiller/ Portable Please let us know if this solved your issue or if need further assistance.