When it comes to creating mockups for websites, Adobe has some great programs for web designers. 
Many designers use Photoshop, but there are some that are also trying Fireworks. In fact, Adobe says Fireworks is good for creating and optimizing "images for the web more quickly and accurately than ever before with an enhanced toolset." As for Photoshop, they say it "delivers features for working with 3D imagery, motion-based content, and advanced image analysis."
By these definitions, wouldn't Fireworks be the obvious choice for a web designer? Maybe, but maybe not. I've used both and find that both are effective tools when designing for the web. Here's why:
Grouping layers is easier with Fireworks:
In Fireworks, if you want to put a bunch of layers in a group together, all you have to do it highlight them on the art board with the cursor, and right-click Group, or apple+G (ctrl+G on a PC). In Photoshop, it's a little bit trickier. You can still select the layers on your art board the same way, but first you need to make a folder for the group. Once you've selected your layers, you then need to drag them in the layer panel to the group folder.
Fireworks has web layers:
Making slices for the web is also quicker in Fireworks. You simply create your hotspot and can then export it for the web. You can also right-click a shape or text and choose "insert hotspot." With Photoshop, there is no right-click option.
Fireworks can save time:
According to this blog post, if you're doing a lot of slicing and exporting, Fireworks can save you about 10 seconds per slice. This may not seem like much, but over time, this can add up. It can also create less tedious work for a web designer.
Photoshop is better for fancy graphics:
If you're designing a more graphic-heavy site, Photoshop is probably the better choice. Since Photoshop is made with advanced image editing in mind, this is probably the way to go for more complex designs.
More people are familiar with Photoshop:
If you need to hand over your design files to a developer or someone else, chances are a PSD file will be more compatible for them than a PNG (Fireworks) document. Fireworks is newer to the scene and hasn't been with Adobe as long (it was a Macromedia product until the two companies merged in 2004). It's never a bad idea to ask up front which program they prefer if you're flexible in which you design in.
Photoshop may perform better:
As this blogger wrote, "Fireworks doesn’t feel as solid as Photoshop. Maybe this is familiarity, but I feel like I’m working from within a piece of carved granite whilst inside Photoshop’s windows. I don’t have that feeling with Fireworks yet, but maybe it will come." I would have to agree with this. However, that doesn't necessarily mean that Fireworks is bad. You may just feel more confident by working in Photoshop because of this.
So, who's the winner?
In the end, I think it really comes down to a matter of preference and what you're familiar with. For now at least, both programs seem to get the job done. I've used both Photoshop and Fireworks myself. My reasons for using one or the other usually come down to what program the company I work for uses.
What do you think? Do you use Photoshop or Fireworks for web design?