The FIRE!
I tried TONS of combinations of Flags like 'Start On', 'Infinite Duration', and everything else. I also tried a trigger to "Start Fire" and then it won't even light. PLEASE HELP.
Basically, env_fire is broken in Portal. The reason it is broken is the fact that Portal's fire pit particle file is called "fire_01.pcf", which also happens to be the name of the HL2 particle file responsible for env_fire.
Because of this identical naming, Portal's file takes priority in the game, and effectively replaces the original. Portal's file does not contain the required particles that env_fire uses to create it's effect, so it doesn't work.
The first way i could figure to fix it was to extract the original "fire_01.pcf" file from "source 2007 shared materials.gcf", and place it in a folder named "particles" in the portal directory. This worked, but broke Portal's fire pit effect in the original game.
To counter this, I renamed the particle file to "fire_02.pcf", extracted the "particles.manifest" file from "portal content.gcf", and added this line:
"file" "particles/fire_02.pcf"
I then placed the fire_02.pcf, and the modified particles.manifest file in the "Particles" folder. This gave me a working env_fire, without breaking Portal's original effects.
The only problem with this approach is the fact that the particles.manifest file cannot be zipped into a .bsp, and must be included seperately. This may lead to your map being broken by other maps which may overwrite this file for their own uses.
Hopefully though, this won't be an issue for much longer. 

