The majority of experienced players will agree that the fire extinguisher is a tool far inferior to the chemical spray, and that its' ease of use doesn't make up for its' reduced efficiency.
The main reason for this, according to my opinion, is the way fire stacks are applied and their damage calculated.
In its' current state, the first fire stack deals significantly high damage, and every other fire stack applied will linearly increase the amount of health a component loses. Fire is strong enough to interrupt repairs starting from a low number of stacks (even more so on balloon) and thus almost always requires urgent attention. At about 10 stacks, half the maximum, the repairing engineer can easily be overwhelmed and fail to maintain the components intact, especially if also receiving damage from other sources (which is more often than not the case). So people are practically forced to immediately counteract fire, which translates into either preemptively using Spray and maintaining it or using Extinguisher once between each repair hit (as 10 fires will easily accumulate on a number of components during the 9 seconds of the mallet cooldown). Due to the above, here's almost no scenario in which usage of extinguisher can prove more effective than spray, as it has a bigger cooldown cost in the wrong run, while also nullifying far less fire damage. There are exceptions of course- their rarity, however, turns Extinguisher from the standard fire fighting tool it's supposed to be to a more situational one, and the 'luxury' Chem Spray takes its' place instead.
Ways to counteract this (if bringing the Fire Extinguisher back to life, past novice games, is a goal)
-Change the Fire Extinguisher. Remove the fire immunity, have it cause no cooldown and be usable on components that are cooling down, but have it remove less fires on application so that the least required time for a full extinguish is relevant to its' animation time
-Change fires. Change the way stacks cause do damage from FirstStackDamage + const * (FireStacks-1) to an exponential function, initially dealing less damage, reaching the same dps as the current one at about 8 fires and dealing much more at 10+ fires. This will allow an engineer to more safely ignore a burning component until it has a number of stacks high enough to be worth an extinguish.
-Change the Chemical Spray. (maybe the most logical one) Redesign its' functionality, making it more punishing if a fight lasts more time than originally expected. Increase its' duration from 20 seconds to 27, and increase its' cooldown from 5 to 9. Spray will now require far smarter timing and include an inherent risk- using it prematurely or entering a prolonged fight will cause it to end at some point midfight. Reapplying it will cost as much as a mallet hit potentially resulting in a dead component due to the other types of damage. Not reapplying it will leave the ship extremely vulnerable to fire. This creates a difficult dilemma for both the engineers and the pilots (both the attacking and the defending ones- should I try and flee or should I continue fighting?) and increases the communication skill ceiling (each engineer might actually have to pick a different tool and alternate roles with each other depending on the situation). Not to mention the fact that a delayed application on a component that's already on fire will be far less rewarding, as using the spray as an extinguishing tool would yield bad results (might actually have to slightly increase the number of stacks removed to 4 or something to make up for this).
