The basic Tau Fire Warrior is, point for point, the best model in the entire army list, and is arguably the best basic Troop in the entire game.  What other basic infantryman has a Strength FIVE weapon?  Add to that a 30" range, longer than most standard weapons, as well as a respectable 4+ save, and these Troops are invaluable.

Take as many Fire Warrior squads as you can fit.  Many folks like to load up on all the special toys, and only take the bare minimum Troop choices, but this is absolutely absurd with the Tau.  At 10 points a piece for the above mentioned advantages, there is no excuse NOT to load up on these basic infantrymen.  In a 1500-point game, have no less than 48, or four full squads of 12.  In lower point-total games, don't go less than 36 Shas'la.  Even with the poor AP of the pulse rifle, the sheer volume of fire and dice rolled will cause even the most heavily-armored troops to begin failing saves.

If you want to take Pulse Carbines, outfit the whole squad with them, and use that unit as a mobile Fire Warrior team.  When on the move, the Pulse Rifles drop to a 12" range, but the Carbines may fire at 18" all the time.  For most of your teams, however, Keep them outfitted with all rifles, as this maximizes their firepower at range.  Taking one or two carbines may seem like a good idea at first to try and cause pinning tests on advancing enemies, but with only a few carbines, coupled with their shorter range, means that you are unlikely to force a test.  Even if you are lucky enough to force the test, it is unlikely that any models in the 40K game will fail it.  Your efforts are better spent getting more shots at 30".  If you want to pin enemies back, use squads of Gun Drones.

Fire Warriors have access to Photon and EMP grenades.  EMP grenades are quite expensive, and those points are probably better spent on other, more effective, tank-hunting weapons such as Railguns!  Having said that, a small team of 6 Fire Warriors with EMPs can be a nasty surprise if they get in place to attack vehicles without getting mauled first.

I would not recommend Photon grenades, as the points spent on these would be better used getting more Fire Warriors.  If your Fire Warriors are getting assaulted, they are almost a guaranteed loss anyway... whether the assaulting Marines get 30 attacks or 20 attacks.  Also, you WANT to lose combat, so that you can hopefully break away and live to shoot another day.

I always like to take the upgrade to Shas'ui for the squad. The extra pip of Leadership is extremely useful, and having the Shas'ui allows you to take a Bonding Knife as well. Always bond your Fire Warrior teams so that they may continue to regroup even when below half strength, and therefore continue to pour fire on the enemy, as well as denying your opponent full VPs for the squad.

With the new Markerlight rules, the 10 points certainly makes it worth it. However, if you intend to mark anything other than what the squad will be shooting at (and they can't benefit from his markerlight), then another 5 points will need to be spent on getting the Shas'ui a Hard-Wired Target Lock. The reduction in points coupled with the improved way Markerlights work make this an attractive option; As the squad guns down infantry, the Shas'ui paints another enemy for, say, the Stealth Suits to mow down later in the phase.

A hardwired Drone Controller is an option. Adding two Gun Drones - especially to a mobile, carbine-wielding squad - help bulk out the squad size, but a Marker Drone with its Networked Markerlight is an excellent choice, allowing the squad to benefit from whatever target it paints. At 30 points, it's a bit expensive, but getting +1 to hit can make all the difference in the world with our BS3 wonders. Shield Drones are a ridiculous choice for a fire warrior squad - it can only try and save one hit a turn, and it costs more than just removing a shas'la. Save the Shield Drones for the Broadside teams.

Taking Devilfish transports for your Fire Warriors really eats into your points.  Your Warriors serve you better standing still and shooting, although in some missions, maneuverability and speed are essential.  Since dedicated Transports do not count as scoring units, the only way they become useful is to either A) Hide until turn five where they zoom in and pick up their squad, zooming away and depositing them in a Quarter on Turn 6, or B) allowing the Fire Warriors to zoom up to a soft enemy, disembark and Rapid Fire the Pulse Rifles within 12". Of course, at that range, if you don't completely annihilate the enemy, that squad of Fire Warriors is toast.  Option C) would be to deploy the fire warriors inside the Fish, zoom it out onto a good firing position in a different table quarter or on the objective, and form your firebase there - but I find that staying still along your own table edge, laying down massed volleys of fire right from Turn 1 is much more successful than any other option. Save the Fish for the Pathfinders.  If you'd like to try a mobile fire warrior squad, though, always take Decoy Launchers to keep your Fish alive on Immobilized results, and a Multi-tracker can be useful to keep the only gun it has still firing when it moves over 6".

The Kroot were billed as the Tau's "close-combat unit."  This is the reason that the Kroot have been getting fairly poor reviews, for they are most definitely NOT close assault specialists.  True, they better at it than the Tau, but they are certainly no shock troop.

The best role for Kroot Carnivores is that of riflemen.  The Kroot Rifle is basically the same as a Boltgun, and they are as accurate as the Tau themselves.  If they can position themselves within a wood, they become nearly invulnerable, as their Fieldcraft ability allows them to see further through woods than any other model in the game.  They can sit in a forest and fire out, and be immune to return fire.  Even in shallower woods, their save bonus means they will last longer than normal as well.

Another good use for Kroot is as a counter-charge force.  Keeping them behind your line of Fire Warriors, when the Tau get charged, the Kroot may pile in the next turn, maximizing their attacks.  If you leave the Kroot in front of the Fire Warriors, the Kroot will get charged, and not receive the bonus charging attack.  If a unit of 20 Kroot gets the charge, however, that can mean 60 attacks at WS4 and S4, so let the Shas'la get charged, and then move in the second-wave Kroot in your own turn.

Kroot are quite inexpensive at 7 points each.  But they are fragile, so you need large mobs... groups of 10 are distinctly ineffective, but a massive squad of 20 Kroot is truly a fearful sight!  Take full squads of 20 Kroot, but I would not recommend the expensive Shaper and armor upgrade.  The Shaper can be nasty in close combat, but you can get 3 Kroot for the price of 1 Shaper.  Quantity over quality is the answer with Kroot. Keep them inexpensive at 7 points, don't waste points on the Shaper and Armor (of 6+, which is nearly useless anyway).

Keeping with the Riflemen theme for the Kroot, add up to 3 Krootox as points allow. Your Kroot won't be able to infiltrate, but they'll be able to take 48" S7 pot shots from turn one - even when hidden up to 12" inside a wood! Their S6 and 3 Attacks certainly help on the Countercharge as well.  I prefer to take more Kroot with Rifles than Hounds that can't shoot - but their I5 means they may take down some enemy before they get to attack - however, your Kroot should be inside Woods whenever possible, and should therefore usually be striking at I10 if they get assaulted.  Having said that, it's hard to beat 2 (or 3, on the charge) S4 attacks at I5 when you consider the bargain price of 6 points.