It could be the work of one of your add-ons, or even add / mal-ware. Look thru your add-ons list and make sure you know what each one is and what it does. Also, check the programs that are on your computer '''Windows:''' Start > Control Panel > Uninstall Programs. '''Mac:''' Open the "Applications" folder. '''Linux:''' Check your user manual. Go thru the list. If you find something that you don't know what it is, use a web search. '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Troubleshoot Firefox Issues Caused By Malware]''' {web link}