The static IP address that you assign must be on the same logical subnet. for example, if your router has established the 192.168.1.x/24 subnet, your DHCP scope will be in the 192.168.1.x subnet range. Your static IP should also be in the same subnet, with the same subnet mask.
If you can access the NAS via firefox, basic network connectivity is correct and working. This issue is an application layer problem (you already validated that you can get to it via a browser). Your test shows that you can get to the NAS using the HTTP protocol on port 80. However, when trying to access it via a UNC path, that uses a different port. You would first need to log onto the NAS and see if you can enable access via SMB.