Credit due where credit is due, and I'm gonna be honest. These screenshots and descriptions are from the official Bradygames guide Limited Edition.
Consider buying it.
There's 30 Firefly Pendants to discover and collect. Since there's bound to be spoilers in places and people, I'll put the pics and descriptions in a spoilertag. It's advised to not open this spoilertag before you've played through the story at least once.
These screenshots is nothing to write home about. If you happen to have one in better condition, I'll exchange it.
*** Spoiler - click to reveal ***
[]Firefly Pendant #1
As Tess and Joel leave the The Quarantine Zone, they use a ladder to enter the second floor of a home. Enter the bedroom on the kitchen's other side and grab the Firefly Pendant from the desk against the left wall. Now you can follow Tess down the steps in the previous room.
[]Firefly Pendant #2
After exiting the warehouse and cutting through an alley, move under a broken-down platform into a small courtyard. Look in the right side of the tree ahead to find a hanging Firefly Pendant.
[]Firefly Pendant #3
Just after meeting Ellie, Joel and her exit the front of the bakery. Before you start your journey with her, search near the corpse across the street for a Firefly Pendant.
[]Firefly Pendant #4
Before entering the Boylstone building (the tilted building), run all the way to the right to the last tree. Look up in the right side to find a Firefly Pendant.
[]Firefly Pendant #5
When you reach the museum's second floor, immediately sidle along the ledge to the left. When you reach the back wall, search inside the display case for a Firefly Pendant.
[]Firefly Pendant #6
Enter the flooded street that leads to the capitol building. Climb into the gazebo in front of the State House. Resting in the flooded waters inside is a Firefly Pendant.
[]Firefly Pendant #7
In the flooded subway section of The Outskirts, you must find a way to get Ellie across the water. Before you do so, swim into the narrow passage on the left. Dive underwater and enter the small storage room to find a Firefly Pendant.
[]Firefly Pendant #8
At the Main Street intersection, an RV sits next to a police car. Use the car to get on top of the camper and move to the rear and find a Firefly Pendant.
[]Firefly Pendant #9
After leaving the bar where Bill led you away from the infected, you follow him toward his hideout at the church. Before getting there, more infected attack in the middle of the street. Once this area is clear, move to the far-right corner in front of the Hardtail Hank's sign. Look up at the streetlight to spot a Firefly pendant.
[]Firefly Pendant #10
After defeating the Runners in the residential alley, turn right and go through the fence. Defeat the Clicker and more Runners before entering the far-right house through a door on the right side. Search the laundry room on your left and find a Firefly Pendant on the dryer.
[]Firefly Pendant #11
Joel and Ellie reach what used to be the Pittsburgh quarantine zone. Hunters occupy the tents in the street and they are inside the bookstore to the right. Move to the alley on the bookstore's far side. At the other end, search behind the overturned furniture to find a Firefly Pendant on the ground.
[]Firefly Pendant #12
After exiting the restaurant and returning to the first floor, enter the woman's restroom through the second door to the left. Search the last stall to find a Firefly Pendant lying on the floor.
[]Firefly Pendant #13
Henry leads you into an office building where his hideout resides. Before following him into SPENzür Architecture, duck into the bathroom on the left. A Firefly Pendant sits atop one of the toilets.
[]Firefly Pendant #14
Drop through the hole in the fishing boat's deck or cut through the big hole on the side. Search underneath the vessel at the bow. A Firefly Pendant is on top of a pile of lobster cages.
[]Firefly Pendant #15
Soon after entering the sewer, climb into the small opening on the right and search in the water on the left for this Firefly Pendant.
[]Firefly Pendant #16
Not long after entering the sewer, when you reach the flooded room with the platform in the middle, turn to the left and jump into the water.Dive underwater and search next to the submerged car to find a Firefly Pendant.
[]Firefly Pendant #17
After searching the house located just before the dead end, exit through the front door, turn left, and step into the side yard. Grab the brick next to the playhouse and throw it at the Firefly Pendant that hangs in the tree above the swings.
[]Firefly Pendant #18
Follow Tommy out of the control room and descend the stairs on the left. Enter the nearby room and search inside the cabinet on the left to find a Firefly Pendant.
[]Firefly Pendant #19
Enter the Hidden Pines Corral lodge and move to the house entrance in the far-right corner of the first floor. A Firefly Pendant is on top of the desk in the corner.
[]Firefly Pendant #20
After exiting the depot and riding up the hill toward the center of campus, turn right and approach the trees ahead. A Firefly Pendant hangs from a low branch of the lighter-colored tree.
[]Firefly Pendant #21
Joel enables the power inside the library and gets the exit open. Go left and before you jump into the courtyard, climb onto the overturned dumpster on the path's right side. Continue into the room above to find a Firefly Pendant on a desk.
[]Firefly Pendant #22
After defeating the Bloater in the dormitory, be sure to loot the body as it drops the 22nd Firefly Pendant.
[]Firefly Pendant #23
In the lab grounds, search the last tent in the far corner for a Firefly Pendant.
[]Firefly Pendant #24
After discovering the source of the noise in the lab and finding the loose monkeys, flip on your flashlight and search the shelves in the room's right side to find a Firefly Pendant.
[]Firefly Pendant #25
While Ellie and David fight off the Clickers inside the building, Ellie knocks a ladder down to David and then climbs up to a walkway. After she eliminates a Clicker, enter the room on the left. A Firefly Pendant is on a wooden crate.
[]Firefly Pendant #26
After Ellie's horse is shot, she enters a cabin at the beach resort. Exit through the front door to the porch. Turn right and enter the gazebo. A Firefly Pendant is on the bench.
[]Firefly Pendant #27
As you descend the freeway exit, run past the school bus and make an immediate left. Hop the rail and search near the orange barrels to find a Firefly Pendant in the rocks.
[]Firefly Pendant #28
Exit the bus depot's waiting room and move past the big triage tent on the left. Find a light fixture just after a portable restroom. Look at the top to find a Firefly Pendant hanging from one of the lights.
[]Firefly Pendant #29
After exiting the triage area and before entering the 611 Tunnel East at the bottom of the hill, circle around the city bus parked across most of the road. A Firefly Pendant is on the other side.
[]Firefly Pendant #30
Just after entering the hospital's sixth floor, use a Shiv to enter the door behind the reception desk ahead. Along with all of the supplies inside, a Firefly Pendant can also be found.