[3.12] You burn = You lose! Burn the world with Firestorm Elementalist

https://pastebin.com/yKqDu8gm

Not taking Holy Fire since it's the lowest value fire damage in range (less than 10% per skillpoint). I didn't play around with clusters yet so there's potential for a decent upgrade there.
Unwavering stance is a late pickup - you can just use the anti-stun pantheon and that's fine but change out of it once you do pickup US.
As for order - just grab everything north of starting position with the exception of shield nodes - you can get those after you get a shield later on since you can just level with two scepters/wands or a staff - then start filling out the left side. Try to grab Constitution before maps as at lower levels life might be a problem.

Chest
Firestorm - Combustion - Immolate - Fire Pen - Controlled Destruction - Inc Aoe/Conc Effect

Gloves
Storm Brand - Hextouch - Flammability - Inc Crit

Boots
Anger - Herald of Ash - Steelskin - Summon Chaos Golem

Helmet
Infernal Cry - Second Wind - Vaal Righteous Fire - Second Golem (can be fire or stone or whichever you preffer)

Weapon
Spell Totem - Scorching Ray - Flame Dash (make sure flame dash is note linked to spell totem - you can also swap places with a golem if it fits your sockets better)

Shield
CWDT - Tempest Shield - Wave of Conviction
Just use whatever level CWDT you feel most comfortable with - I'm running with CWDT lvl 5, Wave lvl 10 and Tempest lvl 10

Just started this lueague so there won't be any op items shown - treat this as a starter point.

Chest

Cloak of Flame is a decent starter item - they are also ridiculously cheap for a 6L - you can get one for half as much as a tabula! They are priced 8c at the time of writing this post. Does not provide life and the ignite bonuses are not very good since we don't scale ignite but the damage taken as fire is very strong - it provides more than 15% additional physical damage reduction (thanks to Paragon of Calamity) and a semi-permanent 40% increased fire spell damage (also thanks to PoC) which makes it a very strong starter item. Later on you might want to switch it to a rare Crusader chest with Enemies you Kill Explode to make your explosions even more ridiculous

Gloves

Rare gloves with life and resists as is common but make sure you grab a pair with additional fire damage against burning enemies

Helmet

Hrimnor's gives armor, life, cold res but also increased fire damage and free/chill immunity as long we've used a fire spell - and we constantly use fire spells so that eliminates the need for a freeze immunity flask - just make sure to cast once before opening a strongbox

Boots

Rare - life, resist, movespeed - nothing fancy

Weapon

Scepter or wand - spell damage in our case is pretty much the same as elemental damage since we don't really care about ignite damage, look for high Fire Spell Damage, added fire to spells and Gain #% of fire as extra chaos

Shield

Rare - Fire Spell Damage, life, resists and anything else you might want - cast speed for example

Amulet

You can start with a standard rare, I would switch to a Xoph's later on for fire penetration and cover in ash (Infernal Cry also does the ash thing so it's not a super high priority)
Ngamahu Tiki is also a decent choice with it's high fire damage

Belt

You can use a rare belt with life and resists or if you feel like you don't need more life you can grab a Doryani's Invitation - just make sure you get the version with increased fire damage - it gives a small ammount of armor as well as more fire leech - all in all a solid belt but you will lose around 300-400 life so choose whatever suits you better.

Rings

You can grab Rare rings with life, resists and fire damage or a Berek's Pass - more damage at the cost of life (it does give some ES which is not entirely useless but still), you could grab a Respite if you preffer but I do not think that the ignite prolif is worth it.
Some honorable mentions:
Ngamahu's Sign - fire damage and life recovery on ignite
Redblade Band - fire damage and life,
Rigwald's Crest - fire damage
What I'm saying is - there's lots of possibilities here.

Jewels
Fire, Life, resists if you need them (you really shouldn't need them here but it's slot you can use if you need), maybe dexterity (you don't really get dexterity from the tree and you need a little bit of it for second wind - not a lot tho', a single low source of dex is enough)

Flasks

1 Instant life flask
Rumi's Concoction (MAKE SURE NOT TO BUY THE REPLICA VERSION!) - lots of armor and block/spell block which is one of our main defenses - Rumi's is a must as it will boost your defenses by a huge ammount
<Free Slot> - choose whatever you like for this one, you do get negative mana regen when standing in place and spamming Firestorm so I added a Zerphi since you don't need a lot of mana regen and I might as well get some more regen while I'm at it - you don't have to use Zerphi, in fact you don't really have to use a mana flask at all but in some cases it may come in handy. If you want a mana flask but not zerphi try to grab an Enduring flask with Cast speed. Grab a basalt flask if you want more defenses - try to get one with a roll for additional armor. If you end up lacking mana regen grabbing arcane will should solve that.
Atziri's Promise - get max elemental as chaos - physical doesn't matter
Quicksilver Flask
Make sure to have bleed immunity on of the flasks since it's probably the hardest hitting ailment in the game, having poison/curse is also handy. We don't need freeze because of Hrimnor's Resolve

Kill All

Ascendancy Order - Pendulum of Destruction -> Mastermind of Discord -> Paragon of Calamity -> Liege of the Primordial

Pendulum gives us either area of effect or damage - both are something we want.
Mastermind of Discord grants 25% Fire Pen - really nice. It also allows us to use a higher % of our mana for some sustained casting
Paragon of Calamity - Damage and Survivability, Reflect immunity is nice tho' situational, Damage leeched as life is very big and allows to even facetank some things (altho' do it at your own risk, some enemies hit very hard) it also synergizes incredibly well with Cloak of Flames granting us more damage and even more physical damage reduction because we almost always are taking fire damage
Liege of the Primordial - well why not? Shaper is really not that useful but if you want to take it - you can. It will slow monster sometimes and increase damage you deal to them via Shocking Conflux, personally I think it's really weak without Beacon of Ruin and since we are not scaling ignite taking BoR is pointless.

Soul of the Brine King - take that before unwavering stance
Soul of Lunaris/Solaris - take either after getting unwavering stance
for the minor I like to grab Tukohama since I'm lazy and like ignore mechanics and facetank and this allows me to die to my laziness a little less. This is vastly prefference tho'.

Grab your Tabula, wanderlust, goldrim and lifesprigs if you have any and proceed to burn everything with with fireball while adding more suports gems as you level - lmp, combustion, added lightning and such - you can easily level this way till you grab your firestorm at level 28. Or if you have some other prefference - go ahead, you can play with the new Blazing Salvo if you want for instance, just make sure you're using a fire skill and you're good to go.

Leveling
Coves T3
Atziri - holy balls I'm rusty, I really need to get used to new atziri
Overgrown Shrine T9
I will add more videos as I progress.

Lots of fire. Lots of fire everywhere. You can oneshot packs with a single meteor, sometimes add a second one for tougher ones, Infernal Cry to clear entire screens - I sometimes even detonate IC with just stormbrand since it is marginally faster in some cases but you can use it however you want.
Fire Penetration total is insance with this setup @ over 170% - you can burn fire elemental made of fire that's how hot your fire is.
Hope you enjoy - cheers.