It could be fire in a bottle... but why use so much fire for this, when light can be substituted and isn't a danger?

[He goes over to look at the glass himself, but refrains from touching, thanks to Shadow's warning.]

And there's no ashes, either... or any sign that there was a fire. But if it was a fairy, then where did it go? Just smashing the container wouldn't have caused it to disintegrate.

And I've never even heard of fairies that put off heat or that would be captured like this. There would have to be at least hundreds.