By the spell's description, Fire Arrows sounds like it would only work on arrows.
However, recent bug fixes show it now working on chakram, shuriken, etc.
Does FA affect all ranged weapons, or just some? What about slings, boulders, etc? Does anything systematic determine whether or not a weapon gets the fire bonus?
Fire arrows works on anything that has range... so slings, javelins, boulders, whatever. Im not sure if it works on natural ranged weapons (poison spit and such) but it works on any weapons that are thrown or shot.
A few patches ago it didn't work on "sticks and stones", the poor monkey's version of shuriken. I thought it was deliberate since they can throw two each round.
Would open some fun tactics if it was changed, but perhaps a bit strong even with horrible friendly fire issues.
It works on any weapons for which there is a fire version:
214 Fire Arbalest
425 Fire Boulder
215 Fire Boulder (Edi's database doesn't show a fire effect for this one.)
217 Fire Bow of War
406 Fire Chakram
356 Fire Composite Bow
213 Fire Crossbow
209 Fire Javelin
212 Fire Long Bow
211 Fire Short Bow
407 Fire Shuriken
210 Fire Sling
405 Fire Small Bow