Folks,

This "problem" with Safari downloading the IPSW.zip files only exists because the default action in Safari is to "Open safe files after downloading. To solve the problem, simply go to Safari-Preferences-General and uncheck the option Open "safe" files after downloading.

There, "Problem" solved. No need to download and use Firefox or any other gymnastics.

*Note: I always set this option as unchecked anyhow. I don't like automatically opening files upon downloading. Bad taste in my mouth from my Windows days