it really depends what area you are clearing. if you are doing ancient tunnels, mausoleum then fire or cold will work. if you are doing chaos then fire will have limited effectiveness. i have tried using lvl 30 conviction + lower res to drop fire res in monsters in these areas but the res is stacked too high. unless you are farming act bosses pvm fire builds are very limited. instead i use either cold or light but almost use light as a main build all the time.

not saying pvm fire ball does not have useful applications and it does. in baal runs if you have high level fb with 20k+ damage + conviction + static the fb min damage is very high so after baal's hp is gone from static you could kill baal very very fast or nearly as fast as a cs zon.