Fire spread can be very damaging to a server. We've all seen the video of the guy building a fireplace and burns down his house.
Once installed, log in game and type /stopfire . You should see a message "Fire spread has been disabled by YOUR_NAME."
You can now light some wood on fire without it spreading to other wooden blocks. Test this in a safe area.
Essentials users: Essentials has a module EssentialsProtect.jar which breaks WorldGuard's stop fire feature. As useful as EP might be, I find WorldGuard to be better. I recommend disabling EssentialsProtect.jar (rename or delete the jar).