Keep using FW CS5 if it works for you.
I can only assume you are seeking a replacement because Apple's latest OSs will not run CS5 applications.
No one EVER states you need the "latest and greatest" if what you have works.
Apple's newer OS's aren't great or worthy of upgrades in my opinion. Again, just my opinion, but I see nothing after OS 10.10 that merits upgrading. So, keep your current OS and CS5.
The "fear of missing out" due to not upgrading has to be overcome.. it's frankly silly and merely profit-seeking methods by developers much of the time.
I always ask myself 2 questions when considering an OS Upgrade...
- What new features do I need in the upgrade?
If the answer is "none" then why upgrade?
- What compatibility issues with existing software will an OS upgrade cause?
This one can be huge. Do I really want to upgrade my OS so it is better integrated with Facebook, Twitter, Instagram, et. al. if it means all my daily-use applications must also be upgraded or replaced? The OS is merely a platform. Anything it does I can do on an iPad or even an iPhone.. why mess with my work center for features that aren't necessary. But that's merely my take on things. I don't strive to be or gain any happiness from knowing I'm on the "bleeding edge" of things if that edge causes more trouble than benefit. In short.. does an iPhone 11 make better calls than an iPhone 7??? Heck no.. so what's the point??
Then there's always a dual boot system if you don't have the "newest" Apple models with firmware lockouts. You can install an OS that works with CS5 and a newer OS. Then merely boot between them as needed on the same Mac or use a virtual machine through something like Parallels Desktop. It inevitably comes down to what works and is easiest for you to use.
If I wanted to dump Fireworks for something newer... I'd be exploring Sketch. I'll point out that I currently don't use Sketch, but from what I've seen it has the highest potential to fill the gaps I would need filled where web content is concerned if Adobe crashed and burned tomorrow. But then, I have CS6 and a system that runs those applications.. so who cares if Adobe crashes and burns.