Old article but still relevant. https://support.mozilla.org/en-US/kb/Firefox+cache+file+was+infected+with+a+virus Whether there is something of a trojan or malware etc in Cache or not, it is harmless while there unless you went in that folder and tried to say run that file yourself. It is harmless if left alone and will be overwritten as stuff gets added to Cache or deleted when you delete cache anyways.