>>64470660
Tbf, a lone Fire Warrior will beat a lone Tempestus Scion in most matchups on the tabletop.
Now, it's true that the Tau have kinda lame accuracy on their own (only as much as an average Guardsmen) but you're supposed to be pairing them up with markerlights which gives them a nice accuracy boost. Plus their range is pretty superb too so they'll be firing well before a Guardsmen gets into range to return fire.
Tau are all about synergy. If Firewarriors are facing Guardsmen without air support, drone support, pathfinder recon and markerlights, etc. something has gone seriously wrong. They'll still probably win but...something has still gone wrong.