Jayne: Six men came to kill me one time. And the best of 'em carried this. It's a Callahan full-bore auto-lock. Customized trigger, double cartridge thorough gauge. It is my very favorite gun.
[he holds the gun out to Mal]
Mal: [exclaims in Chinese] You offering me a trade?
Jayne: A trade? Hell, it's theft. This the best gun made by man. It has *extreme* sentimental value. It's miles more worthy 'n what you got!
Mal: What I got? She has a name.
Jayne: So does this. I call it Vera.
Mal: Well, my days of not takin' ya seriously are certainly comin' to a middle.

I'v only seen the first episode and I can't remember who said it but it goes something like this...

"If we're lucky they'll rape us to death, eat our flesh and sew our skin ito their clothes, and if we're really lucky, they'll do it in that order."

Wash: "Can we go to the town where I'm a hero next?"
and for some reason Wash again: "Put down your arms or this little moon gets a new crater"

Wash: "Its grotesque! Oh look and theres something in a jar."

Wash: "Its ok, im a leaf on the wind"
Mal: "What does that mean?"

damn you people youve gone and quoted the whole show. and yes this has been done. but i like the time mal repeats the "tell niska that we dont want his money" line to the second soldier.

Mercy is the mark of a great man.

*stab*

..guess I'm just a good man.

*stab again*

.. well, I'm alright.

Doesn't really count but.

"Gorramnit Mal! .... Forgot mah line!"

Sejs Cube:
Mercy is the mark of a great man.

*stab*

..guess I'm just a good man.

*stab again*

.. well, I'm alright.

hahaha. i second that :)

Niska has captured Mal and is torturing him. The crew of Serenity are loading up with many, many guns to go and get him back.. including Shepherd Book, who is in the process of handing out firearms and loading his own weapons while this exchange takes place.
Zoe: Don't the Bible have some particular things to say about killing, Shepherd?
Book: That it does, Zoe, but the Good Book is a mite fuzzier on the subject of kneecaps.

Leodiensian:
Niska has captured Mal and is torturing him. The crew of Serenity are loading up with many, many guns to go and get him back.. including Shepherd Book, who is in the process of handing out firearms and loading his own weapons while this exchange takes place.
Zoe: Don't the Bible have some particular things to say about killing, Shepherd?
Book: That it does, Zoe, but the Good Book is a mite fuzzier on the subject of kneecaps.

And later in that episode:
Zoe: Wait. This is a fight the captain needs to finish for himself.

Mal: No he doesn't!

Zoe: Oh.

In Heart Of Gold where they save the...umm... "House of ill repute".
The gang is talking about going and helping the people.
Zoe" No one's going to force you to ge Jayne. As stated this job is strictly speculative".
Jayne " Good. Don't know these folks, don't care to ".
Mal " They're whores ".
Jayne (immediately) " I'm in ".

LMAO so funny.

I'm trying do decide exactly how pathetic it is that I want to correct some of these posts....

Righteous One:

"If we're lucky they'll rape us to death, eat our flesh and sew our skin into their clothes, and if we're really lucky, they'll do it in that order."

Zoey said it.

I find it hilarious every time Mal and Jayne ruin a dramatic, tense scene by shooting someone.

It wasn't technically in Firefly or Serenity but it was in the outtakes during Serenity when Mal has the idea of making their ship look like a Reaver ship, he comes out of Serenity and says something like

Mal: Ok, we need a hood ornament, Kaylee, get that kid who's taking a dirt nap with baby jesus, and Jayne! Stop stealing their stuff!

I'm gonna be in trouble for this one aren't I? o_o

Mal: Jayne. Go play with your rain-stick.

Simon: You know, these are better than the [I can't remember the last name]'s berries.
River: They are. Except they're poisoned.

Melaisis:
I find it hilarious every time Mal and Jayne ruin a dramatic, tense scene by shooting someone.

Like near the end of the second episode, when the fed has River hostage, Mal and co are coming in hot, pursued by Reavers. Mal doesn't even blink as he shoots the fed right between the eyes and dumps his body out for the Reavers to eat.

Saffron: He's my husband.
Mal: Well who in the damn galaxy ain't?!

Monty: I SHAVED MY BEARD FOR YOU, DEVIL WOMAN!

Mr. Universe: He killed me Mal, he killed me with a sword. How weird is that?

Mal: Do you wanna run this ship?
Jayne: Yes.
Mal: Well... You can't!

Objects in Space Episode:

Jayne: You sayin' she's a witch?!
Wash: Yes, Jayne, she has had congress with the beast.
Jayne: She's in congress?!
Wash: How did your brain grasp the human language? Seriously, I'm curious to know.

Safe Episode:

"dear diary, today i was pompous and my sister was crazy. today we were kidnapped by hill-folk never to be seen again. it was the best day ever!" -Jayne Cobb

Serenity (pilot episode):

Jayne: "Testing, testing. Captain, can you hear me?"
Mal: "I'm standing right here."
Jayne: "You're coming through good and loud."
Mal: " 'Cause I'm standing right here."

Heart of Gold:

Jayne: (calls out from across the room while hanging all over a whore) "Can I start gettin' sexed already?"
Mal: "Well, that was kind of horrific."

Our Mrs. Reynolds:

Bandit: You gonna give us what's due us. And every damn thing else on that boat. And I think maybe you gonna give me a little one-on-one time with the missus.
Jayne: Oh, I think you might wanna reconsider that last part. See, I married me a powerful ugly creature.
[The "woman" looks up - it's Mal in a dress.]
Mal: How can you say that? How can you shame me in front of new people?
Jayne: If I could make you prettier I would!
Mal: You are not the man I met a year ago!

I'd go on, but, then I'd just end up quoting the whole series.