I think its heyday was when HTML table layouts were still how you had to layout websites and it's slicing capabilities were really convenient. I suspect that was the killer feature for a lot of people but once CSS layouts took over, Fireworks really fell out of favor and Adobe never bothered to reposition it as a UI tool.

I still use it to this day for my limited editing/mockup needs. The new workflow seems to be mockup tool (like Balsamiq) -> Photoshop design -> HTML/CSS, but for my needs, I can get reasonably close enough in both layout and design in Fireworks just do Fireworks -> HTML/CSS.

I don't know if a tool that is close enough to Fireworks to be a good replacement has come around, but I know it's definitely not Photoshop, which I find far too complicated for the basic tasks I need.

I've gone from Photoshop (1st version was 4.0 that I remember) -> Fireworks -> Sketch