The lightning entity does create random fires on impact for server and the client.
By chance the client creates fires without the server knowing about them.
These fires are not affected by rain, don't spread and don't hurt enties since they don't exist for the server.
The client shouldn't be allowed to create these fires on it's own. The fires created by the server get send to him anyways.
1. Go into a biome other then desert, hell or sky.
2. Turn on a thunderstorm (/weather thunder)
3. Wait a few minutes and look out for fires which are not affected my the rain.
Video of these fires: http://www.youtube.com/watch?v=cSFhVL_ES3M#t=0m47s
This post maybe means the same bug. But since the one I posted has nothing to do with doFireTick I'm not sure about that.