Sorry for the double post, I thought up some things to say about the fire.
The 'sort of transparent' effect is nice. Keep it. The fire itself though, looks too watery, if that makes sense. The little details inside it aren't jagged enough and look more like bubbles then the effect you were going for. The shape is also a bit messed up, fire doesn't shape itself that way. As i said in my post above, I can't really describe how i do it, and i don't want to try and end up giving bad advice, so well, i wont.
But i can say, don't have any transparency in the middle of the fire, but nearer to the outer edges is where you really want to show off the effect. The sparks around the fire don't make much sense, to get a more realistic (i know you want to keep the cartoony style) effect, have the sparks get smaller the further away from the fire they get. Also, you probably want to make them a bit more jagged, to avoid that watery effect i mentioned earlier.
I see it as 'don't tell them what they did right, but what they did wrong'. Its a good drawing overall, but you'll never get any better if everyone just tells you it's good, or says something like "It just looks off". So yeah, sorry for the wall of text, but i think it should help if you take it to heart.