Is there a mod that'll allow for my Sim to set random things (and people) on fire that still works?

Ummm... okay... that's, ::gulp:: that's great, for you, definitely, a great way to, um, spice things up. Sure. ::backs away very, very slowly::

Just kidding, you're fine. We all know it's the silent ones who are the most dangerous. ::glances around furtively::

In all seriousness, I was really disappointed with the hidden pyro trait (sometimes inherited from a high-level firefighter parent). While you can click on the sim in question and prompt them to (words to the effect of) "light something nearby on fire," you never get to choose which item, or "item," as the case may be. Even worse, the fire dies down with absolutely no damage to the surrounding area, and in less time than it takes the fire department to respond. And with not so much as a crazy-eyes pyro moodlet! As a test, I had a sim sit in a burning chair, and she didn't even notice, even though she most definitely was not immune to fire—I just used nraas MC to give her the pyro trait. I know EA has the teen rating to contend with, but how about teaching children that their actions have consequences?

On the other hand, you could just abandon a stove mid-roast like everyone else.

Demonic powers still works I think, or at least there are no comments saying otherwise on the MTS page.

You can start fires with witches in supernatural too.

If you have the Pets expansion pack, you could get your hands on a Unicorn - they can start (and extinguish) fires.

With the University Life expansion pack, daring a Sim to divide by zero may set the receiving Sim on fire.

Mods-wise, I think NRaas DebugEnabler allows you to start fires on any patch of ground you click on. Click on the terrain, then choose NRaas -> DebugEnabler -> Options: -> DEBUG: Add Fire.