It's not a real technical term. Remember, you read it in a newspaper, the foremost propagators of misinformation. Most likely what was being referred to are network sniffers. Unless of course that journalist knows someone with a really weird fetish.
Edit (resulting from the question edit)
To the extent that knowledge is power network, sniffers and port scanners (another possible explanation of "firewall sniffer") are indeed a threat. i.e. Finding an open port or protocol based vulnerability is a problem because it presents an attacker with a definite attack vector.
As such tools are common place, and most of us use them for various purposes, anyone managing firewall and other security devices must work on the assumption that the "bad guys" also have those tools. One of the more legitimate uses of those tools is in fact testing our own firewalls to determine weaknesses, with the intention of fixing any that may be found. Someone wanting to attack them could reasonably be expected to do likewise.