The No Firewatcher Mod removes the default behavior named firewatcher which sends rain every time that fires get too out of control. It also includes performance changes to fire mechanics (makes the game run faster/smoother). In addition, the rain amount now affects the amount of damage done to fire (only takes place with custom weather).
Using the No Firewatcher Mod you won’t need to use the Improved Fire Performance mod anymore. This mod provides greater performance as the firewatcher also consumes time (and IFP does not remove the firewatcher). However, feel free to use both — it will cause no errors.