There are a couple of pictures in the book, one for a Firenewt by itself, and another for a more heavily armoured version on a Giant Strider. I like the former picture, which has a nice colour palette and an endearing expression on the Fire Newt's face, but the other one is a little flat. Both do a good job of showing the proportions and size of the creatures though, so they are successful.
So the Fire Newts adhere pretty closely to the 'crazy militant church' trope, with a love of sacrificing people to Imix, taking no prisoners in time of war, and having a culture that is, in general, based around worship of a big angry fire dude. I don't think that many other D&D races go in for this kind of crazy - the Drow obsession with Lloth is more abusive in tone than this - but it feels a bit staid to me. Probably because I have gotten a lifetime supply of this trope from playing Warhammer.
More interesting to me is the ecology and 'lair' description. These guys need both heat and moisture, so they seek out hot springs, boiling mud, and the like. They delve down and dig out a home in the area, and then turn to alchemy, blacksmithing, and other crafty pursuits, despite being of low intelligence as a race. They construct sluices that carry hot liquid around the settlement, for which I am imagining boiling water constantly gushing around the corners of rooms, a bit like internal guttering. This all sounds like a basis for a visually strong dungeon, similar to some of the Fire Cultist ones from Princes of the Apocalypse, and certainly helps to differentiate them from, say, Kobolds. The entry makes no mention of whether they work with other cultists of Imix, but it is safe to say that allying these guys with other Fire-themed stuff, especially drawn from the bestiary at the end of Princes, will serve you well. There is also some really beautiful Mike Shley maps for those cult areas, which by themselves will serve you well.
So here we get three statblocks: a CR 1/2 Warrior, a CR 1 Giant Strider, and a CR 1 Warlock of Imix. I'm a big fan of how they use Warlocks to represent 'priests' of various nasty things like Krakens and Elemental spirits, and it allows you to sidestep concerns over deities being big bad guys. Note that all of these lads are immune to fire as well, which is as it should be. Anyway, let's talk about these three quickly.
The Warrior is pretty tough for his CR, I think. Decent AC and HP means that he won't be going down easy, while his (melee only) attacks are very respectable. He has a rather fun Spit Fire ability, derived from alchemy not anatomy, which is a 2d8 Fire damage (Dex 11 for half) that they can use once a fight. That seems pretty hardcore for CR 1/2, to be honest, and I can see these guys earning real respect from the players. Being a Tiefling with Fire Resistance will pay dividends against these guys!
The Giant Strider is mainly notable for the near 100% chance that one of your players will try to tame and ride one. It's essentially a two-legged horse that regains HP when hit by fire attacks (e.g. a Warrior's Spit Fire) and can do a half-sized and half-damage Fireball (with a normal Dex 12 save for half, and recharges on 5-6). Wait, now that I think about it, that really isn't very similar to a horse at all. The biggest issue I see with them is the very wonky rules for mounted combat in 5e - especially the way that mounts die instantly to AoEs at high level and oddness around the rider and mount action economy - but when ridden by equally low HP critters like the Fire Newts that shouldn't be a problem.
Finally, the Warlock is fairly simple to run: he gets always-on Mage Armour, he can see through the Darkness spell, he casts Fire Bolt normally with the option for 2 castings of various fire spells (Burning Hands,Flaming Sphere,Hellish Rebuke,Scorching Ray) that have various ranges and effects. For a 'Fire warlock', that is actually a pretty well-rounded selection. He is pretty much exactly what you'd want and expect a low-level 'Fire' caster to be, I think, and it will be very easy to steal this statblock for a human Shaman of the Flames or whatever - especially as you can use swarms of them if needed, since they are only CR 1. This low CR means that Fire Newts have no high level entry, unlike say a Goblin Boss or an Orc Warlord; I'd recommend, again, going to Princes of the Apocalypse for that need: in addition to CR 4 Fire Priests and the like, there is Vanifer, the Prophet of Fire, who should be easy to convert into a Fire Newt, and her CR 9 statblock is engaging and high enough level to serve as a fine BBEG.
Overall: though their theme is slightly tired, these guys bring fun statblocks, interesting lairs, and a lot of synergy with opponents found in Princes, meaning that you'll probably find a use for them long after Goblins cease to challenge your players.
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