I am the owner of my own server but can't even start fires. I looked in my essentials config and I enabled fire-flint (or whatever it is) and it still doesn't let me start fires. If you need any more info, please let me know what. I'm a noob, sorry.
Just post the what plugins you are using ect. I know there's a problem with Pex that if you op a player, they can't interact with everything like they could with say the operator permission node, that may be the same for group manager but I can't be sure.
1) To disable or enable the lighter use the WorldGuard config for a world, not a flag on __global__, that's not good practice. 2) Allowing the lighter or not is irrelevant regarding fire spread. You can easily permit lighters, but still fire spread would be nerfed, quite a typical setup for WorldGuard, actually. This allows for stuff like a fire place in a house, without torching half the map.