I believe Adobe Muse is the Fireworks replacement; however, I am only speculating. I do not use Fireworks, and I am sure I wont miss it if this is the case.
There was a statement made in May 2013 as to the future of Fireworks.
I hope this is helpful.
For me there simply is no replacement for Fireworks CS6 within Adobe's toolset or outside, so I'm still rocking away with it for day-to-day design. Certainly Muse is no comparison, it's like the HTML features of Fireworks that I never use, expanded further. Some people love it, though... to each their own.
I hope that Adobe creates a new true web-design tool like Fireworks.
Fireworks CS6 is the last version. From Wikipedia:
On May 6, 2013, Adobe announced that Fireworks would be phased out, much to the dismay of its users. Adobe will continue to provide security updates and perhaps bug fixes for the current version, but does not plan to add any new features beyond what is in Fireworks CS6.
Muse is a completely different application: basically a wysiwyg web design app for designers who do not want to or are unable to code html pages. Muse can also be a very nice prototyping app, though.
But currently there is not any true replacement yet on the market - I suppose Adobe is trying to turn Photoshop in a more web-friendly tool (which I think is only semi-successful). If you work on a mac Sketch might be a partial alternative.
This recent article suggests some general alternatives such as Sketch, Macaw and Anetype.
Other possible replacements for Fireworks include:
Evolve UI (Open Source) by Tribaloid
Tribaloid | Engineering creativity
Gravit (currently in Beta and available by invitation)
If you continue to use Fireworks today, you will be interested in Fireworks extension resource sites such as these:
As a long-time Fireworks user and fan, I would agree with the above comment that nothing either inside or outside Adobe has managed to fill the void left by Fireworks. But some of the programs above do look promising. I think Adobe leaving Fireworks CS6 in "maintenance mode" and available to Creative Cloud users is an acknowledgment of that.
I have found Adobe Muse useful for proto-typing web sites.