First and foremost let me preface my hypothesis by saying that it's been a fun ride watching the Red John chase from the very start and I hope the ending will not be unsatisfying.

The next three episodes are going to flow like a movie, and my theory for "Fire and Brimstone" is that Jane believes he has collected all his clues he needs to identify Red John, with the final Red Tattoo clue being the big discriminator. What Jane does not know however, which the audience does, is the whole Tyger Tyger situation.

From the promo Jane has got all 5 suspects in the room at gunpoint to finally get the moment he has been waiting for for the past 10 years-to catch Red John. What I believe will happen is that Jane, almost in a extremely simplistic way by his standards (albeit somewhat rational), will order them all to take off their tops to reveal which has The Red Tattoo thinking that whoever does must be Red John. However, the twist that I am predicting is that MOST of the 5 will have that tattoo if not ALL of them. With the tattoo relating to the Tyger Tyger organization in some way. This will initially stump Jane, but be his first insight into finding out about the vast secret organization leading into the 7th episode (The Great Red Dragon), and delaying what he thinks will happen in "First and Brimstone" but will actually occur in the 8th episode, which is the reveal of Red John.

These 3 episodes are the climax of 6 years of storytelling. It's gonna be good. :)

A few other points:

Many believe that Red John is not any of the 5 or even initial 7. I personally believe Bruno on this matter, however anything could happen.

Something more important for me just as much as who Red John is, is a thorough explanation of EVERYTHING that has transpired during the Red John storyline, I.e the He Is Ma clue, how and why do his disciples do what they do. After 6 seasons, the Red John storyline needs to conclude not only entertainingly, but with good clarity into all the clues and red herrings, not only in these S6 episodes, but the whole series. I think the fans that have stuck by deserve that at the least, and laziness in the final chapter would be a shame.

The theories will be coming thick and fast now!