Borderlands: From Claptrap's New Robot Revolution: It's one of the things Rebel Claptraps yell.
Borderlands: The Pre-Sequel!: In the mission "Eradicate!", President Tassiter has you assemble a new Claptrap that strongly resembles a Dalek to fight an infected soldier. It's called a CL4P-LEK, and sounds just like how a Claptrap version of a Dalek would.
Firefly: Multiple things of the series is referenced:
One mission has you kill Jaynis Kobb. You even find him in Jaynistown.
Referencing River Tam, with the "Firefly" class mod for Lillith, resident supernaturally-powered girl who specializes in close-quarters combat.
Borderlands: The Pre-Sequel!: A line from Jack in the beginning, when he and the Player Character meet The Watcher for the first time, when asked what the, at the time unnamed, Watcher is:
"Uh, no idea, but I get the feeling it could kill us with its brain, so, y'know, keep running!"
Borderlands 3: The Companion is a twofer - It deals fire damage on headshots, mimicking the Firefly perk from Destiny... and has a quote to complete the reference. Similarly to Captain Mal's gun, it's also a revolver-styled pistol that acts more like a semiauto (and visually looks just like Mal’s gun):