First access this page and sign in with your Adobe credentials.
Then you can go to the ProDesignTools page and download the Fireworks CS6 offline installer
You need to visit the Adobe page and login first to set a cookie in your web browser before you can download. The links are genuine direct Adobe links and safe.
If you have a valid license key it should still activate Fireworks. Asuming you are on a Windows PC it should be OK, The newest versions of Mac OS that do not support 32 bit apps will mean you can't install/run Fireworks. A few Fireworks users on the forums here have recommeded Parallels desktop to run a Windows virtual mahcine on Mac Catalina or Big Sur and run Fireworks that way, I haven't tried it myself but I've seen good reviews.
If for any reason you are unable to download a Fireworks installer let me know and I will help you.