In the main event of the promotion's biggest card of the year, featuring four title bouts on New Year's Eve at The Theater at Madison Square garden in New York, Gaethje held on to his title via TKO after three rounds when their brutal main event was stopped by the cageside doctor.
Firmino (19-8), 34, took the fight on three weeks' notice to replace the injured Joao Zeferino and was leading on all three scorecards when his right eye was grotesquely swollen shut. The native of Brazil pushed the pace off the start and appeared to land the bigger shots throughout the first two rounds.
But the gritty Gaethje never backed down, despite appearing weary in Round 2 as Firmino began to tee off with strikes. But as the faces of both fighters began to bleed and bruise, it was Firmino's that was overcome with swelling.
"I did three good rounds and I think I won," Firmino said. "It was a great fight and Gaethje is a tough fighter."
Gaethje (17-0), who lost his mouthpiece four separate times, forcing referee Dan Miragliotta to threaten to take a point, did well to stuff takedowns in Round 3 and answer Firmino's pressure with heavy counter shots.
"I guess they stopped it because he couldn't see, but hats off to him because he came to fight," Gaethje said. "I love the competition and I love entertaining people. I came out here and gave my all. I'm a little emotional, but I'm an emotional dude and that's the way I fight."
The victory was the 10th straight for the unbeaten Gaethje, 28, since his WSOF debut in 2013 and the fifth defense of the title he captured in January 2014. Gaethje, who trains out of Denver, said he wasn't surprised by Firmino's aggressive pace.
"We're fighters, that's what we do. He better want to swing back," Gaethje said. "Our health is on the line. We were both in here fighting for our lives and fighting for glory.
"I wished I could have finished this a different way. I think I was in the process of breaking him. But like I said, hats off to him. That was fun. I love this."
Firmino pointed to his chin afterward and asked for a second fight.
"I know he [punches] heavy but nobody knocks me out," Firmino said. "My chin is strong. You can punch and do whatever you want. I train hard, I'm a veteran and I know what I'm doing. I want a rematch. For sure I can beat this guy if I train two months. It's a big difference. I know I can beat this guy."
Fitch hints at retirement after decision win over Shields
After the fight, he brought up the possibility of retirement.
Fitch (29-7-1), 38, a former UFC title challenger, referenced recent medical issues, including a bulging disc and "a funny reading on my brain MRI" that left him considering walking away from a 14-year career.
"I've been dealing with a lot of medical s--- over the years and there's a good chance this is my last fight," Fitch said. "I'm still grappling with retirement. I have to talk to some doctors and some neurologists and see what they can do.
"They've come a long way with stem cell injections, so with the degeneration I have in my spine, if I can get back to grappling and actually having fun grappling like I used to, then we'll see."
Fitch received the nod from the cageside judges via unanimous decision (49-46 on all three scorecards). He combined with Shields to suppress much of the bout's potential excitement as the 25-minute affair was largely contested on the ground as Fitch routinely stuffed takedowns to grab top position and Shields threatened with submission attempts from his back.
"Once you get into a heavy grappling match back and forth with Jake, it slows you down a little bit," Fitch said. "That's what happens when you put two top grapplers in there together."
Fitch made the first defense of the vacant title he won in April and improved to 5-2 with the promotion after leaving the UFC in 2013. Although the fight was largely void of striking, Fitch confused Shields by coming out as a southpaw.
"He had a great game plan and came in there southpaw and I never saw Jon do that before," Shields said. "He stuffed my takedowns and kept his distance. He was in phenomenal shape in there tonight."
Shields, 37, a former Strikeforce middleweight champion, snapped a 17-month layoff since his last appearance, when he lost a welterweight title bout to former champion Rousimar Palhares (who was fresh off a victory over Fitch).
But Shields wasn't willing to blame cage rust on the loss.
"Hats off to Jon on a great career. If he retires, he's one of the top five welterweights of all time," Shields said. "I don't want to make any excuses -- this was Jon's fight tonight. If he doesn't retire, maybe we'll fight again, but he was just better tonight."
Fitch fought off a late heel hook attempt in Round 5 and finished the bout as the aggressor, a role he played throughout the five rounds.
Silva withdraws due to injury
Moraes (18-4) ran his win streak to 13 fights and showed in the opening two minutes why his name is considered among the top 135-pound fighters in the world. After rocking his opponent early with a looping right hand, Moraes landed flush with a two-punch combination and followed it up with a right hand to floor Silva (25-5).
The Brazil native swooped in with a series of hammer fists before sinking in a guillotine attempt. Silva worked back up to his feet and scored with a knee to the body.
But after Moraes slipped while attempting a kick, Silva went down with him and instantly recoiled due to pain in his right knee. Referee Dan Miragliotta stepped in to wave off the fight at two minutes and 30 seconds.
"You saw, I hit guys and they go down," Moraes said. "I feel like I'm one of the best -- if not the best -- bantamweight in the world. I can hang with any 135-pound fighter in the world. Just put them in the cage and sign a contract with my name and I'm going to show up ready."
Branch defends WSOF middleweight title
Two-division WSOF champion David Branch closed out an ambitious 2016 with a dominant victory in front of his home fans.
Branch (20-3), a native of the Bronx, made the third defense of his middleweight championship with a fifth-round submission win over Louis Taylor (13-4). The victory was the 10th straight for Branch, who added the WSOF light heavyweight belt to his collection in September 2015 by claiming the inaugural title.
"A lot of people have been saying I'm the best 185-pounder not in the UFC," Branch said. "I think I'm the best 185-pounder in the world regardless of promotion and I'm planning on proving that in 2017."
Branch controlled the early rounds with his wrestling but ran into a bit of trouble in Round 3 when Taylor secured a tight guillotine choke from his back. Branch worked free before later escaping a standing version of the choke against the cage.
The champion rebounded to dominate the final two rounds, recording clean takedowns and scoring with short elbows and punches from top position. Branch secured full mount in the final round, forcing Taylor to give up his back as Branch secured the rear-naked choke that forced him to tap at two minutes of Round 5.
"I really didn't take any punishment," Branch said. "Yes, he was tough, but I got the win and I'm looking to do big things in 2017."