There is no replacement by adobe. They announced a successor some years ago, but nothing happend until today.
Fireworks is dead and it died in 2013
FunnySheep is correct: Fireworks is dead, dead, dead. No more official support, and no more updates are forthcoming.
I suppose Photoshop and Illustator are Adobe's idea of an replacement. The new Comet app might also be an option, I guess, but that is more aimed at prototyping, I believe.
Otherwise, you will not find any true alternative on the market as of yet. For Mac Sketch and Affinity may be a good alternative. For Windows I like Photoline which I work with nowadays (extended it with its external app link), and use it in combination with other tools. Xara comes quite close as well.