firmware download usb directory firmware/nos2.1.0
It's not. Drop the 'firmware/' in the path. Correct and working (from 2.0.0 to 2.1.0 at least) is:
firmware download usb directory nos2.1.0
I believe you need to have a Brocade USB. Perhaps it works with other sticks with the same folder layout, I don't know.
And by the way, there are no release.plist file in the root of the nos2.1.0 folder.
Brocade USB folder layout - one folder 'brocade' in the root and four subfolders: