Faster Firing is the first upgrade on Path 2 for the Sniper Monkey. It costs $340 on Easy, $400 on Medium, $430 on Hard and $480 on Impoppable. It increases the attack speed of the Sniper Monkey, allowing him to fire exactly 1.4 times as fast. It is overall a relatively useful upgrade because non-upgraded Sniper Monkeys attack quite slow. This Sniper Monkey upgrade changes the DPS of the Sniper Monkey by about 10%, which may not seem like a huge difference, but it is. Adding this upgrade to about 15 Sniper Monkeys can make them do a lot more damage!
In BTD6, this upgrade returns as Fast Firing. It also appears as the first upgrade of Path 3. It costs $280 on Easy, $TBA on Medium, $TBA on Hard, and $TBA on Impoppable.