So in the last few days I have received countless messages of people asking why their downloaded .ipsw doesn't work. I have also seen the same question asked on the forums in many many threads. The vast majority of the time it is because they used Safari to download the .ipsw and Safari decided to go ahead and extract it.
If you are using Safari and want to download a .ipsw follow this guide to change a setting and everything will be good.
Go to the menu bar (top of your screen) and Click Safari.
Go to General.
Uncheck the Open "safe" files after downloading box.
Close preferences and find your firmware here and download it.
Yes, after you spend 15 seconds unchecking a box in Safari it never unchecks itself and will allow you to download firmware for the foreseeable future.
Note: Safari will probably label the download .ipsw.zip. Delete the .zip