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 to 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 does not break it; it must hit the floor. This also applies to the Tank, in which a direct throw of the Molotov at the Tank does not burn it, it must be thrown on the floor instead.
 * 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 incapacitated, if not worse.
 * Molotovs can bounce off walls slightly, giving you a chance to run away from the flame or throw it around a corner.
 * Generally, refrain from throwing Molotovs in front of you, unless there is a Tank, or a Horde. Most likely in some scenarios where the area is engulfed in fire will 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 difficulty, even Survivors with full health can die from the flames.
 * Setting a Witch on fire will gradually burn her health away and make her move extremely slow (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 Deads 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 the said weapon is rather risky, as it takes quite some time to kill, but in playing Left 4 Dead 2, burning the Witch then running with Adrenaline makes killing a Witch much easier.
 * Infected AI does not attempt to bypass the fire in any way. If you know where a Horde is coming from, throw the Molotov at a choke point between the Horde and your teammates. 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 will travel in a straight line unless a static object is blocking the way. The fact that Infected seem to surround the Survivors is due to the AI counting other Infected as an obstacle to get around.
 * 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. A 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 a 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. This is most likely, for gameplay purposes, as if the water causes the flames to extinguish, a tank/horde will be free to kill the players easily without any more molotovs being thrown at them. (only if there's only one person with a molotov)
 * Once lit, Special Infected cannot 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 southeast 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 Common Infected 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.
 * Throwing Molotovs on ensnared players can release them, as the flames burn the Infected grabbing the player.

Infected

 * 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 explode forcefully, 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 cans 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, and 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. Also in Dead Center, using a Molotov or incendiary ammo on the Uncommon Infected, Jimmy Gibbs Junior, will have no effect on him due to his fireproof racing suit.

Behind the Scenes
An early screenshot shows that the Molotov was gray and had a different-shaped top before becoming the final design. It was also intended to be a usable weapon in Valves other game, Half-Life 2, but was cut from that final version of the game before being introduced into Left 4 Dead.