It didn't warrant putting in my proposed edit to the Pyromaniac section of the "Traits" page, but after testing each lit building and going through RimWorld's code via the dnSpy decompiler, it seems to work like this:
Whenever it needs to determine how many stacks it has, the thought checks all cells within 8cells of the Pyromaniac's position. If said cell is within map bounds, isn't in the fog-of-war, and is within the same room as the Pyromaniac, the thought then adds however many fires are in that cell to the amount of stacks it has (up to the max of 4 stacks). The method it uses to determine how many fires are in a given cell is weird. It grabs every 'Thing' in the cell, and checks if...
1) it has a component that is/derives from "CompFireOverlayBase" (meaning that, under some circumstance, the Thing will display a fire graphic overlaid atop it), that it has a "CompGlower" (under some circumstance the Thing will emanate light), and the CompGlower is currently glowing.
2) the Thing currently has a fire attached to it.
3) the Thing is Fire, and it is attached to an object.
For each such condition that is true, the cell is considered to have +1 fire.
Onto the weird exceptions in how the total mood impact of "Beautiful fire" is calculated:
- Lit burnbongs do not count as fires: burnbongs are the only building in unmodded RimWorld which DO have a derivative of CompFireOverlayBase, but DON'T have a CompGlower. They thus don't fulfill condition '1)'.
- Free-spreading fire counts as 2 fires: the Thing which caught fire fulfills condition '2)', and the fire itself is a Fire Thing, which fulfills condition '3)'. This feels unintentional.
- Free-spreading fire on a Pawn counts as 1 fire: ... I thought I had an answer for this, but on reading over the code again I have no clue why this is.
- Burning walls don't count: they're not part of the current room.
- If a building occupies at least one cell within 8c of the Pyromaniac, and it is on fire, BUT that fire is only in cell(s) further than 8c from the Pyromaniac... it will still be counted by condition '2)' since it is, technically, on fire. In fact, each cell of the building within 8c of the Pyromaniac will count as an individual passed check for '2)'.
- Due to similar per-cell logic, a building that fulfills '1)' can do so multiple times if it occupies multiple cells. This is why pyres, being 2x2, can provide up to 4 stacks of the buff.
- A doorway counts as its own room, so a Pyromaniac standing in a doorway won't get any stacks even if there's a fire directly on either/all side(s) of the doorway.