I am not sure even after seeing the picture from the 'rule book' that came with the YV-666 Expanison Pack that can that ship fire behind or not? It sure does look like it could, but then my friend diagrees that it would be like that. He said it fires only on front and sides..
Please clarify this for me!! 
Your friend is correct. The shaded area on the base indicates the firing arcs, and they project out to the sides.
It fires 180 degrees forward. The two 45 degree arcs off the front are auxiliary arcs, similar to the rear arc off a firespray. (IE things -are- in arc).
In the Package for the YV-666 there is a Booklet that clearly outlines what constitutes as the fire arc of the ship in which it comes with.
I am not sure even after seeing the picture from the 'rule book' that came with the YV-666 Expanison Pack that can that ship fire behind or not? It sure does look like it could, but then my friend diagrees that it would be like that. He said it fires only on front and sides..
Please clarify this for me!!
The picture in the booklet is highlighting the Auxillary arcs only (the Primary arc & non shooting area are faded out) he fire 180 degrees out the front & as said above the base card has a slight orange tinge in the arc areas.
If you have access to a Slave1 booklet have a look in it & you will see that the picture is only highlighting the rear arc beacuse these pictures are under the Auxillary section
In the Package for the YV-666 there is a Booklet that clearly outlines what constitutes as the fire arc of the ship in which it comes with.
It's actually the booklet that's created the confusion. The shading/highlighting on the diagram is what has confused many folks. The ship's token is the clearer example.
I had thought it could fire behind it but now I see it can only fire in the front 180. Thanks for the clarification.
In the Package for the YV-666 there is a Booklet that clearly outlines what constitutes as the fire arc of the ship in which it comes with.
While you feel that it "clearly outlines" it... for you and some folks. Other folks, such as the poster, it was not so clear. Different people learn different ways, which would be a reason why these Rules Questions forums exist in the first place. Clearly. 