As a lvl 40 fire/fire blaster I have a few suggestiong and points to make. Overall I'd say your assessment was good however.
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Ring of Fire: In the early levels, I relied on this quite a bit to keep foes out of arms reach. Later levels, I very rarely use this. I didn't bother throwing any extra slots on it, and many times any enhancements in it are red.
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As far as ring of fire goes it is indeed not the best power in the secondary pool but in a pool that lacks good powers (and being a power that you forced to have) I would not negate the possible importance of it. In a high lvl senerio, for example a crey tank, ring of fire can come in handle. I have it two slotting one hold and acc.
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Combustion: Ok, this one I'm not overly sure of.
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To clear this one up let me just state DO NOT TAKE THIS POWER. There are alot of people that will disagree with this, however, a blaster is only ever two hits away from death. In order to use combustion you must be really close. You never want to be that close.
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Rain of Fire: I have seen a lot of people recommend to bypass this
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And I would be another one to do just that. In saying that, Rain is an ok power and if you really want it that is fine. However, rain will make your tank and controller teammates upset (since it will awake mobs and fear mobs away from tanks) plus if you solo alot rain will simply make it much more difficult to use you AOE's. Fire blasters are not single target blasters and with Rain that is what you become. If you use rain as a "Oh crap" weapon I would be willing to bet that you'd still be better off having the mobs in a tight area to use fire ball and fire breath.
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Fire Breath
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I think your missing the point of the fire blaster here. The best combo for a fire blaster is Aim+Buildup+Fire Ball+Fire Breath. Use that combo and you are a blaster master

. In fire breath I have 5 damage and 1 acc. This is a PRIMARY weapon and a must have. Now some people might comment that fire breath should come first in this combo. Either way will really work most of the time, however, fire breath has a high tick damage count which is why I use it second. Using fire ball first will normally get me the first hit a damage (which is the most) and one tick. If I click fire breath once fire ball is used, Fire breath will then get the three tick damage which is more than what fire ball would get in three ticks.
Some other general comments:
1. Get buildup much much sooner. As a matter of fact get build-up as soon as possible. Use three recharge enhancements in it and use it as often as possible.
2. Aim is a very nice compliment to buildup. I have two recharge enh's in it and that seems to work well. This is also a must power.
3. Blaze and Blazing bolt are also great powers to have. Blaze's range is poor but its damage is great. Great power to use on red's and bosses. 5 dmg and 1 acc. Blazing bolt is also a must have (same slots as Blaze). This is a great power to startup a fight with. Use stealth and SS so that you are invisible get close to the boss, hit blazing bolt on the boss the fire ball+ fire breath and the mob will fall.
4. Inferno: Inferno is that ultimate power in the fire/fire blaster set. Inferno will take out a group of purple minions on one shot. When I got inferno I used it alot while hunting in Bricks (mainly the crey groups in the lower left corner) and it did not dissappoint. I slotted inferno with 3dmg 3recharge and that allows me to use it on every other mob. Now, the endurance cost is great so you either have to carry around end with you or simply get consume. Consume is another power that people just stay away from for some reason. But inferno+consume is a great combo.
5. Burn: I like burn. Burn is great as a defensive weapon. Mobs will not enter melee range (plus it is a good way to not zzzzzzzz). I just have the initial slot with dmg.
Hope this helps some. Just my opinion really but I find it works well. Good luck.