I’ll plan a bigger a post on this when I’m not busy, but D&Ds fire newts being a militaristic society feels like it wouldn’t necessarily work in every setting unless they have a good defensive geographic stronghold. 

Personally, I think the best way to do them is to make it so they need hot springs or artificial equivalents to breed (as 5e seems to be doing), and the societies are built around protecting these spawning grounds (which they do not seem to be doing). Some may turn to evil gods that are good at causing fear for such means of protection, and that can spiral into an evil society, helped by a disdain and lack of empathy for life not heat resistant. However I expect earth shaping magic to also be a big part of their way of life.

Their raiding culture as written does make sense if they are a secure civilization have a low infant mortality rate ; its a response to having more individuals than they know what to do with. And so they send them out to gain glory (and thus prestige and mating opportunities) or die. 

If they drive too many prey/marks/sacrifices populations away from within easy reach of their home though, they might turn to more “legitimate” methods such as adventuring and mercenary work.

There’s also more economic methods of gaining the resources they need to protect their way of life.

What I’m proposing is: fire newts not part of empires actnig as the owners of hot spring resorts. They allow use the hot springs that aren’t suitable for spawning due to either relatively low temperature or poor location for safety concerns from predators. The money is then funneled into magical defense of settlements deeper within their territory away from the resort.

tldr: Firenewts who have secured their settlements against threats have two options: either practice population control or declare endless war against outside. And since they live around volcanoes they can’t be easily rooted out.