Fire looks better in 3D viewport then in render? When I take an object and click "quick smoke" and set the flow type to fire, it looks very good in the viewport. However the render looks terrible. Here is an example:

Viewport-


Render-


It, for some reason, has a black outline. I saw this problem much more when I too the monkey, set to to quick explode, then added a smoke domain. The viewport looked so much more better then what the final result looked like. 

Also, is there a way to make the fire not fill up the domain and look like a square in the final result?
The viewer is broken regarding images with alpha, and composites them incorrectly. Never trust it. Never rely on it.

Composite your work using the Alpha Over node against an image and you will have an accurate rendering of your fire.