It's a very short prequel, not even a novella, more like a novelette, so my review will be proportionally short as well. *grinning*
My advise is if you haven't read Firelight already, pick up this book first and see if you like the style of the author. If you've read Firelight already I don't think you should bother with Ember. It tells you the same things you found out about Archer and Miranda from Firelight already, but in a little bit more detail and chronologically up to the point of Archer's return to London.
So as I said, it's unnecessary to read Ember if you read Firelight already.