When I first heard about Frostfire Bolt, I was not impressed. Don't get me wrong, my Fire-specced friends would have killed for a spell that deals frost damage to fire-immune mobs when we were raiding Molten Core back in 2005. The spell comes with a weird compromise of Fireball's base range (but NOT range extension from talents), a 3 second cast (talented fireball casts at 3s, while talented frostbolt casts at 2.5s), and BOTH Frostbolt's useful chill effect and Fireball's detrimental damage over time effect (rarely enough damage to make a difference in the fight, but just enough that you can't resheep your target if you get your targeting backwards).
Little did I realize the full destructive power that can be had by conjuring up a little water, freezing it into a large snowball, setting it ON FIRE, and hurling it at one's enemies.
My mage just hit 75, so I'm working with rank 1 of the spell (the level 80 version does about 100 more damage). Sitting at around 1100 spell damage with an appropriate 48 fire/18 frost build, it's good for about 2K per hit and somewhere around 4500-5000 for a crit. (By comparison, I was doing about 1800 regular/3600 crit on regular frostbolts as a deep frost spec.) What makes this spell fun is that it benefits from talents that enhance BOTH Fire AND Frost spells. At level 75, you can cherry pick the best of both worlds.
On the downside, you don't have either a water elemental (which I do like immensely), or ice barrier (which is nice because the frost build is pretty mana efficient, and having a PW:S equivalent means not having to take damage either). What do you get for giving up some goodies? Well, I decided to take a real fire build for a spin for the first time ever to give it a shot.
Crispy Fried Eagle
The victim that I found was a hapless eagle in the Grizzly Hills. What the massive chunk of combat log spam I've posted above doesn't convey is the sense of time. Let's break it down.
t=0: I cast frost nova to freeze the soon to be crispy fried bird, and incidentally dealing 636 damage. Under ideal circumstances, the bird might be frozen in some other way, such as via Frostbite, but that's the start of the combo.
t=1.5s: Global cooldown is over, I used the time to back out of melee range and promptly start casting Frostfire Bolt.
t=4.5s: As this time approaches, I start mashing the button for the instant cast fireblast, thus the note in the log that I failed to cast the spell a second time because it had already gone off.
t=4.50001s (or whatever): The Frostfire Bolt has a travel time and the Fire Blast arrives instantly, so both hit our hapless frozen bird. Enter Shatter. I'm looking at a 24% base fire spell crit rate (haven't been stacking crit rating, and the stuff I have has been degrading as I leveled), plus an extra 50% because the bird is frozen. The Fire Blast crits for 2357 damage, while the Frostfire Bolt crits for 4417.
t=4.50002s (or whatever): Now my talent procs kick in. The bird is set on fire with ignite, so he should be in store for another 40% of that 6.7K pair of crits over 8 seconds. I get a lucky impact proc, which is a 10% chance on any spellcast (not just fire anymore, it's great for Frost mages too these days), so he's also stunned for 2 seconds. He's also re-frozen by Frostbite (the crits broke the original freeze) for 5 seconds. And, to add insult to injury, I get back 30% of the base mana cost of each spell from Master of Elements (even though I probably got a discount off of that base number via talents), AND a [Mark of Defiance] mana proc for good measure.
t=4.50003s: As if all that weren't bad enough, I just got two consecutive crits with qualifying Fire spells, which means it's time for Hot Streak to proc.
t=6s: I cast an instant cast Pyroblast for 2.5K damage courtesy of Hot Streak. (The pyroblast failed to crit even though the bird was frozen via frostbite, since 74% chance to crit frozen targets is still more than 1 in 4 of failing to crit.)
The hapless bird has now taken almost precisely 10K damage over a 1.5 second window (which took me a total of 6 seconds to set up). He's also ON FIRE in as many as four different ways (DOT components of the Frostfire Bolt and Pyroblast along with the two ignites). Unfortunately for Mr. Bird, what he's already taken was 569 more HP than he had.
No, we didn't light it, but we tried to fight it
This all may sound impressive, but it gets better. The Hot Streak proc itself only lasts for 10 seconds after the second crit, but you appear to have a nigh unlimited time between the first and second crits. This means that you can wait for frost nova to cool down if your previous fight ends with a crit. You'll also note that I have, but didn't use, Combustion in this example).
The point being, these procs aren't a certain thing, but they're sure enough. I ran the Violet Hold with some guildies and had no trouble at all burning down the elite ads (which, like our friend the hapless eagle, have about 10K HP). I certainly take more damage and have slightly more downtime solo, but it's not a bad deal, especially in a group setting with a quality tank and healer. Turns out that Frostfire bolt is more interesting than I thought.