Photoshop has become the de-facto for graphic design. It has always been that way and it continues to be so. Fireworks though now a product of Adobe was originally created by Macromedia and launched in 1998. Fireworks was created with functionalities similar to Illustrator and Photoshop and was no doubt created for screen design. Fireworks strength has always been its simplicity and its ability to allow us to do rapid prototyping.
Fireworks since its first release always suited best for website design and since its last 3 releases, Adobe has made better integration between Fireworks and other software in the creative suite specially Photoshop & Illustrator.