Is that the name given? Firefox can’t find the server at www.(xyz).com 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.