Nothing, really. I just abandoned 1:1 pixel design all together, because that's how I used Fireworks. I would do everything based on pixel ratios between objects. It kind of defined my art style since everything was geometric to some extent. Now I'll just sorta eyeball everything in Photoshop or Illustrator.
Affinity Designer seems to actually get kinda close to the OG Fireworks, at least on the surface, but I haven't really used it much, just casually played around with it.