Check this link: https://www.bing.com/search?q=searchitnow

I've gone through all those steps in that link more than once over the past two or three months. No "suspect" files showed up anywhere. I couldn't find anything to delete.

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.