Fire warriors aren't the major concern, though they do throw out a ton of 30" range S5 shots--it's the pathfinders and other sources of markerlights that are the real combo backbreakers in a Tau list. Almost all Tau units are BS3, but markerlight hits are the way they boost that, and also ignore cover, etc. Remove the markerlights and you can nearly halve the effectiveness of Tau shooting.
Other units to look out for:
- a "Buffmander," or a commander who gives just about every special rule a shooting unit could want to whatever shooting unit he joins. High decapitation priority.
- Broadsides. Specifically, Missilesides who throw out something in the neighborhood of a fistful of S7 and a fistful of S5 Ignores Cover dice, all twin-linked, each. And this squad of 3 will have Split Fire. It will melt about 2 unit per turn, no trouble. Kill. These. Guys. (They may have 2+ armor, if I recall correctly).
- Riptides. Big nasty MCs, now accompanied by Stormsurges, which are bigger nastier Gargantuan MCs, that can overcharge their shooting for a risk of hurting themselves to lay down about double the pain. There are also some insane Forgeworld variants of Riptides that are insane--for instance, one has a Heavy 2 AP2 flamer with 6" torrent, and also a S8 Heavy 3 (or Heavy 3+D3 if nova charging) Haywire gun that can jump like a Flying Monstrous Creature swoops and can jump out of combat into ongoing reserve. Just... by way of example.
- All the formations. Tau got SO MANY incredible formations from the Kauyon and Mont'ka campaign books that I can't even list them here, but you might be facing 3-9 basic Riptides that can shoot twice and reroll failed Nova Charges; masses of drones with Jink, Interceptor, Split Fire, Skyfire, and boosted BS; stealthy units with mega-buffs against shooting that hit the rear armor of whatever tank they shoot at regardless of facing; and any one of a number of other formations.
Eldar... have a very powerful codex in their own right, so best of luck, but Tau have a lot of nasty tricks up their sleeve, and a lot of possible builds. Forewarned is forearmed.
Thank you for the detailed response! Lots of good info in there.