Go to the official windows download page without a windows useragent and you get the iso instead of the crappy download tool.
And amusingly this isn't an easy task in Firefox.
This is only for responsive design mode in which FF mimics iPhone (Mozilla/5.0 (iPhone; CPU iPhone OS 12_0 like Mac OS X) AppleWebKit/604.1.38 (KHTML, like Gecko) Version/12.0 Mobile/15A372 Safari/604.1)
There is no straight way to just change the useragent like I could do with IE10 *grin*