While the technological principles exist- we've seen explosives, bombs, fireworks, metallurgy, and large guns (as well as a variety of projectiles driven by explosions which are ostensibly non-gun guns... like the net-guns or shocking bolo disks fired from the mecha suits)- firearms would serve limited practical purpose in their world and culture. As an honor culture, long range sniping would be frowned upon. It is completely ridiculous for any modern political conflict to be resolved or addressed by single combat, but we still get a one-on-one duel between Kuvira and Korra to decide the fate of a nation state!
Whatever utility you might get from ranged firearms is sacrificed as breaking a fundamental rule of society through dishonorable combat (possibly a reason that Combustion Man was on the fringes of society rather than hailed as an exceptional bender to learn from an emulate; see Toph by contrast). Even if bender have an advantage it's one that has to be refined through kung fu (transliterated literally as "skillful work"), your combat competence is earned. This is why chi-blockers, or Jett, etc. are acknowledged despite no bending their ability is a hard-to-develop skill.
Additionally as a highly collectivist (Zaheer) and hierarchical (Amon) culture, personal autonomy isn't as valued and the wild-west fantasy of self determination with a firearm isn't a thing (or at least it's addressed through bending, like Wan forging off on his own; only to resume societal duties after his wandering). Meaning you rely on the police and military and state for defense, not yourself most times.
Finally, realize that firearms existed for centuries before becoming the primary personal weapon. Despite existing in concept many technological innovations had to occur to make them sufficiently reliable, standardized, and effective to justify putting down the sword and spear or aside the bow and arrow. The central and primary one is quality control, interchangeable parts, mass production, and cost... all things that they have in the form of industry in Legend of Korra but all tied to all the cultural things mentioned already above.
A lone wolf might be able to hand craft a function firearm of OK performance and occasional reliability, but for it to really be a prolific device you need industry behind it. Unless Varrick or Asami want the stigma of completely undermining their honor culture, they're not going to formalize the firearm, which means you're not going to see it on the streets even if in some abstract concept it's a utilitarian effective implement. Just because you can argue it's good for a specific situation doesn't mean it's good for the culture or the businesses that have to decide whether to bring it into being or to market.