In past threads, this problem was caused by malware, but I can't recall whether anyone named the specific programs that caused it. To do a general clean up, you can use the tools listed in our support article: [[Troubleshoot Firefox issues caused by malware]]. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.