I have recently submitted a query because I was unable to uninstall Butterscotch toolbar. I had tried all the usual ways but to no avail, however, I have now resolved the problem. I use AVG 2011 antivirus software and found the ofending files in C:\Program Files\AVG\AVG10\Toolbar\Firefox\avg@igeared\chrome\content\libs where there were 2 files include.js and include_lite.js. There was also a Toolbar.old\Firefox\avg@igeared\chrome\content\libs containing the same two file names. I deleted the Toolbar folder and rename Toolbar.old as Toolbar and the problem disapeared. I don't know why, you are the experts, but it may help others with the same problem.