The best build for FIre TB, IMO is this:
1. Fireball
2. Circle of Fire
3. Fire Aura
4. Fireball 2
5. HP
6. Fire Nova
7. Fireball 3
8. Circle of Fire
9. HP
10. Fire Nova
Then work from there.
Favor Item-Blood of the Fallen
Buy 550 Armor, then 1500 boots, 1750 armor, 1750 helmet for mana and 3300 gloves.
If the game last long and you have the cash, sell one of our armors and buy 10000 armor and mage slayer. However, its rare games even last that long anymore.
You won't get a ton of kills, but you will be able to take down towers early which is super important, especially if opponents haven't got Tower HP yet. The key is to time your circle of fire to be used on tower and as creeps are incoming so you take their HP down enough that your creeps take them out easily and you can also move in to take down tower while creeps attack so your HP isn't taken. Same goes for when you use Fire Nova. Why waste on just creeps, when you can use to help take towers down. ANother tip if you want to take out towers faster, once you've used circle of fire, switch to Ice TB briefly and your Ice damage increases for a short period by about 80 hit pts. Switch back and your mana regen picks up briefly. Use a few times and the tower will be down in no time.
Fire Nova is awesome. I like to go mid on Cataract when opponents creeps are coming in and right before they make their turn towards the lanes, I take both sets of creeps out. If you do it just right, you will also do 400+ damage to both front towers. This allows your creeps to head straight to corner towers and work on them if they aren't down yet or gives your teammates one less thing to worry about in the lanes. This works especially well if corner towers are down and you have priests. If they got priests too, even better because now you've taken out the enemies ability to use their HP regen while they fight.
Like I mentioned, you won't get a ton of kills, however, if you are a good teammate, remind your teammates to chase when needed because your fireball will do a ton of damage on the run. You will get plenty of assists which are just as important as kills. And because you HP stacked early, as long as you aren't dumb, you won't die very often, which is an extra bonus. If you encounter a melee DG, always use Circle of Fire first, Nova then Fireball. That little bit of damage early when they are in close, may just be enough to take their damage down so when they run, your fireball takes them out.
Late game, with this build, you will have over 5000 HP, which doesn't seem like much, but you will also have over 5000 mana which more than makes up for your lack of HP.
Remember, your goal as a Fire TB is to take towers out as early as possible and assist your teammates, by helping with their kills or taking out large groups of creeps to clear paths.