To edit the fires, you need to go to 141 in unLZ, however if you just want to change the color of the flames then use Advanced Palette Editor. The palette you need to edit is the first row in the sprite box if you go to palette viewer in visualboyadvance when you're at the title screen.
In case you're not familiar with how to use advanced palette editor, once you open load the ROM, click on the radio button that say "load by searching" and then go to the tools menu and click on color picker. then use the color dropper thing to find the gba value for each color in the palette (in order) and put those values into the boxes that say "Actual Palette" in the main APE window. Then click Load once, then click copy once. The colors will appear in the bottom section labeled "Changed Palette". You can then change the values of any of the colors down in the changed palette section and when you're done click "Replace" next to the pallet loading options and it will write it to the ROM.