Molotov Cocktail

"Molotovs, hell yeah!"

- Louis "Fire in a bottle baby."

- Ellis

The Molotov Cocktail is a throwable weapon that engulfs an area with flames for approximately 15-17 seconds. The area of effect is quite large and spreads over time before self-extinguishing. The fire will burn all Infected that come into contact with it. While Common Infected cannot attack after being ignited and die in seconds, Special Infected can withstand the fire until their health is burned away. The Molotov cocktail is dangerous in close combat because the splash damage and flames may cause you and nearby teammates to lose health. While it is burning, it can ignite other combustibles such as the gas can, the propane tank, fireworks, and the oxygen tank.

Survivors

 * When a Common Infected is on fire, neither the Infected itself or the fire on it will damage you, and water will not extinguish the flame. A burning Common Infected is generally considered as dead(bots will not shoot them), but the fire can obstruct your sight and distract you.
 * Throwing a Molotov at a wall or a Tank does not break it; it must hit the floor.
 * In versus games especially, conserve at least one Molotov for the Tank. Make sure it hits, otherwise, don't be shy in using them to stop hordes that come from behind you. However be mindful of the position, if your team is using shotguns in a wide open area, and a wily smoker can pull someone past the flames, they're most likely incapped, if not worse.
 * Molotovs bounce off walls slightly, making a chance to run away from the flame or throw it around a corner, not exposing yourself.
 * Generally refrain from throwing Molotovs in front of you, unless there is a Tank, horde, or other serious threat. Most likely scenario is that you'll cause your team to wait it out, giving the Infected and special Infected some well needed time to spawn. Worst case scenario; the Molotov someone in your team threw in front of you means death to one or more of your team, due to needing to wait in a hard to defend spot.
 * Do not hit teammates. If they are incapacitated inside a fire, it may be difficult to revive them and on Expert, even Survivors with high Health may be killed by the time the Flames die down.
 * Where the Pipe bomb is less effective (e.g. against Special Infected) due to its fuse, the Molotov shines. Setting a Witch on fire will gradually burn her health away and make her move extremely slowly (unlike the Bile Bomb, which you should NEVER use on a Witch unless far away), making her easier to kill. A burning Tank will survive for 30 seconds on normal, 40 on advanced, and 45 on expert. Only in Left 4 Dead's campaign and survival modes, setting the Tank on fire will increase his running speed, since he doesn't have much time to attack the Survivors.
 * Burning a Witch with said weapon is rather risky, as it takes quite some time to kill, but when playing Left 4 Dead 2, burning the witch then running with adrenaline makes killing a Witch a walk in the park.
 * No Infected of any kind know better than to avoid fire and will gladly cross it to reach you. The crux of the Molotov strategy is to let them. If you know where a Horde is coming from, throw the Molotov at a choke point between the Horde and your teammates. The flaming barrier will burn all Common Infected as they try to cross it. Try to throw the Molotov just as they reach the choke point in order to increase the likelihood of burning the entire Horde before the flames expire. Note that sometimes they will try to find a way around the fire, but it is not likely. Infected tend to want to get from point A to point B in a straight line unless a static object is in the way, like a big tree trunk.
 * Notice that in Versus, human players may avoid fire and attack Survivors from other side. Boomers may try to walk through fire to ambush the Survivors and make himself a ticking bomb, while Hunters can ignite themselves to cause extra damage. Witch that runs through the fire will always switch target.
 * Obviously, fire does not burn in water, although in real life alcohol fires can burn in water. However, if you throw the Molotov at the edge of the water(on riverbank), it will still burn normally regardless of the water.
 * There is at least one exception to this: Death Toll's first Crescendo Event in the storm drains. A Molotov thrown into the shallow water will burn as normal.
 * Also, despite the way water affected the molotov in Left 4 Dead, it does not seem to affect them at all in Left 4 Dead 2. The many water-filled areas of Swamp Fever and Hard Rain can still support molotov fire. Even the heavy downpours in the Hard Rain campaign cannot douse the flames.
 * Once lit, Special Infected do not self-extinguish. Unless the map has water on it (e.g. Death Toll Finale). They will burn until they die. This applies for Tanks and Witches, too.
 * In Versus, if a player is hanging and being attacked by a Smoker out of reach from normal weapons, consider lobbing a Molotov to release them, if you cannot even attack the tongue.
 * Since the Molotov will not set off car alarms, a common strategy is to lob a Molotov at a car alarm to kill any Smokers that would try to pull you towards the car (which will set off the car alarm).
 * When on the Rooftop Finale in the No Mercy campaign, when you get to the ladder, you can, rather than just lobbing a Molotov onto the roof, climb the ladder and check where the Infected are. Usually there's a Smoker south-east of your position. Lobbing a Molotov there from the elevator shaft should get rid of a Smoker, thus leaving a Boomer and two to three Hunters(in L4D Versus) on the roof to deal with. This takes a lot of practice and should be practiced on in single-player with cheats on.
 * A very risky tactic is, when you are surrounded by CI and need to get away fast; throw the molotov at your feet and run, the fire will kill any common infected surrounding you. This is refered to as "Ninja-Toving" by some players. This should never be attempted on expert and only in advanced if you have green health; in other difficulties this only does about 5-8 damage to you.
 * Try to avoid throwing Molotovs in close proximity to your team, otherwise you will have to wait until the flames die down until you can pass/double back. Furthermore, Special Infected can utilize the Fire to deal heavy damage to you such as Smokers pulling you through the fire or Hunters utilizing the Fiery upgrade to pounce you while you are stationary, especially in Versus Mode.

Infected

 * When lit, you can not go into ghost mode due to some players being unfair and attacking with a Hunter then running back into ghost mode.
 * Though it will eventually kill it, the Hunter does greater than normal damage when it is on fire. The bonus fire damage is only gained by lighting a Hunter on fire using a Molotov or Gas can.
 * As a Boomer, you can set yourself on fire to become a ticking time bomb. Run into Survivors to forcefully explode, since the Survivors can't safely push you away and shoot you without the risk of being exploded on. However, Boomers don't have very much health, so don't set yourself on fire too early.
 * If you're playing as a Smoker, a great tactic is to drag a Survivor through the flames, causing massive damage and preventing others from rescuing them. Take care as they can kill you with their weapons; flame doesn't burn bullets.
 * In versus, as a Tank, the Survivor team will immediately attempt to light you on fire. You must do your best to avoid the Molotov or Gas Can they set up. If you are stuck in front of a choke point, you'll often have no choice but to run through and be lit. Either look for a way to extinguish your flames from water, or failing that, waste no time running at the Survivors and inflicting as much damage as possible.
 * A good tactic for the Jockey is to try to ride a Survivor through the flames. This will cause flame damage and cut the lone Survivor off from the rest who will have to wait for the flames to stop in order to save their companion.
 * When playing as the Spitter try to spit just in front of the flames where the Survivors are standing. The acid will cause them to panic and try to escape in any way possible, this will cause some players to unintentionally jump into the flames causing massive damage and confusion.
 * As the Charger try to take players through the flames to the other side. This will hurt you, but it hurts the Survivor as well and will leave him or her cut off from the rest of the group. Hitting any additional Survivors is a bonus, as if they land in the fire, they will receive massive damage before they are able to rescue their teammate, or even before they can stand up!
 * Predict where they will throw it through the use of environment. If you think they will throw it there, move somewhere else.

Left 4 Dead 2
In Left 4 Dead 2, during the campaigns Dead Center and The Sacrifice, using a Molotov or incendiary ammo on the Uncommon Infected, the CEDA Worker Infected, will have no effect due to the hazmat suits they are wearing.

Behind the Scenes
An early screenshot shows that the Molotov was gray and had a different-shaped top before becoming the final design.