Do you want to enable it? In which case it would be /res pset firespread t or disable it by typing /res pset firespread f Here is the list of commands that are available to us http://empireminecraft.com/guide/commands
I 'think' firespread is entire res based so use /res SET firespread t or f pset is for individual perms
Actually it is /res set firespread false (if you type pset it will not work) EDIT: Mystul beat me to it
Just a warning: if you disable firespread, you disable grass's ability to grow on your lot. If you're farming sheep, put out your fireplaces/lava and turn it back on : )