I hear you pyromancers out there clamoring for me to talk about you. Frost had the more pressing need! It's the only spec to majorly change in patch 5.4. The only change to fire was a 50% nerf to Combustion because - let's face it - it was just too good. Even after the nerf, fire is still really, really good.
But I'm new here, and we haven't talked about mages in quite awhile, so let's take a long view of fire. Prior to Mists, fire saw a lovely and elegant redesign. Fire has always been my spec of choice. Maybe it's the visceral WHOOSH-BOOM that accompanies a Pyroblast! proc, or the satisfying crackle of flame at your fingertips. Whatever it is, you have to admit - fire is the sexiest mage spec.
A brief history of burning things
Mists changed fire in a few fundamental ways. First, it introduced the Heating Up mechanic to even out some of the RNG aspects of playing as a fire mage. By allowing you to "save" a crit to furnish your next Hot Streak, the design made it so that you'd seldom be penalized by either wasting a Pyroblast! or failing to get one because you didn't crit two times in a row. Pyroblasts on demand are a pretty heady feeling. There are some fights where the procs come hot on each other's heels and you can barely cast them fast enough.
The other thing new to fire was Inferno Blast. Rather than fishing for Impact procs, Inferno Blast gave a reliable way to spread Combustion DoTs as well as a guaranteed crit, adding some needed control to fire's cleave capabilities. They even made it so that Combustion resets the cooldown on Inferno Blast so that you are guaranteed to have it available right after casting Combustion.
Combustion remains fire's biggest cooldown and source of both pain and joy. When glyphed (and it's recommended that you glyph it) with Glyph of Combustion, it gives you a major damage boost every 90 seconds. The downside of this is that with a longer cooldown, flubbing a Combustion will hurt your damage more than it used to. Fire in its current iteration isn't perfect, though. I miss having very strong AoE damage and I don't find "put bombs on everything" to be a really engaging playstyle to achieve it. It's also a small thing, but I really miss a Blast Wave/Flamestrike combo.
Still, fire is overall one of the most fun specs to play. It has the perfect mix of mobility and a steady rotation but enough reactivity and careful planning to keep it interesting. Every caster class has some degree of filler nuke, and you'll cast plenty of Fireballs. But there's a strategy to tracking and lining up the perfect Combustion, casting Pyroblast! procs quickly, and if you've thought ahead - spreading your Combustion to another target on fights where it's an option.
Keys to success as a fire mage
Because of fire's high barrier for entry, many mages might just now be looking at trying the spec out. There probably won't be much that's new to fire mage veterans in this article. But because of fire's tendency to become awesome at the end of expansions we're going to discuss some key fire points as more mages are lining up to set things ablaze.
First of all, you should have a gear level higher than about 535. I'm not one for making hard limits, of course, do what you want to do - but 535 is a good approximate benchmark to approach the kind of crit you'll need to really see fire shine. For this reason, fire isn't a great spec to pick up on a mage alt you barely play or a character that has just dinged 90. It's fire's ongoing quirk that it performs poorly at lower gear levels, and usually by the end of an expansion requires nerfs because it scales so well with gear. If you try it at low gear levels, you're liable to be disappointed. If you're determined, you could gear a fresh mage to the needed gear levels through a combination of Timeless Isle items and Burdens of Eternity.
You'll want to aim for at least 40% crit chance. Crit is fire's lifeblood and without it, you'll have a hard time getting the Pyroblast! and Ignites that you need. Once you find yourself cackling with a slightly giddy tone because you just procced four Hot Streaks in a row and it's like someone brought you a puppy and a mug of hot cocoa AND it's your birthday, you'll know you're doing it right.
That elusive Combustion
You must have a way to track Combustion and your procs. The in-game notifiers for Heating Up and Pyroblast! work fine, but you can also disable them and use WeakAuras or another similar addon if you prefer. The majority of fire mages will be using CombustionHelper to track their Combustion. CombustionHelper is a customizable fire mage addon that actually has some other features built in if you choose to enable them. The addon is pretty self-explanatory but it does take some getting used to if you're not sure what you're looking at.
These three examples of CombustionHelper's windows show the absence of a Bomb, Ignite, and Pyroblast in that order. You'll also note that the middle window has turned yellow. This is to let you know that Combustion will be available to cast quite soon, 13 seconds in this case. The box will remain yellow once Combustion is available if you are missing either Ignite or the Pyroblast buff. Your bomb doesn't benefit Combustion, but you should have as near to 100% uptime on it as you can regardless.
You'll also notice that the window has turned green and all of the buttons and numbers are also green. When all of these stars align, cast Combustion for great justice and watch your enemy go up in flames. If there are any secondary targets nearby, use Inferno Blast to make sure they go up in flames, too.
After you've cast Combustion, the bar at the bottom switches over to a timer letting you know when you can cast your next Combustion. At the default setting the box will also go transparent. I keep my CombustionHelper pretty transparent because I put it right over top of my character and I still need to be able to see what's happening behind. By default it is quite small, but you can easily adjust this in the configuration.
This is the real crux of fire and part of the RNG that makes it so rewarding (and at times, frustrating). The variability of your Combustion will determine to a large extent how successful you are in any given fight. It's a little bit like playing the stock market. There's a good possibility that you can get a higher Ignite, so you might hold out - but you could also lose the Ignite that you have and end up with less than you had in the first place. Whether you find this mini game fun or aggravating depends on your temperament.
Obviously I haven't covered everything about playing a fire mage here. This Icy Veins guide is the most comprehensive and helpful I've found for an aspiring pyromancer. It includes an opening sequence, tips for how to use Alter Time, and even macros to help you do so. Not included are tips specifically for setting warlocks on fire, which I'm obliged to mention. That's covered in Fire Maging 102. I'm sure you'll figure it out, though!