I think people get way too nervous about the OS their computer is running -- and Canon, with the wording on their downloads pages, aren't really helping.
First, remember that the firmware you're downloading runs on your camera, not your computer. The computer is only used to get the firmware file -- usually called "something.FIR" -- and copy it to a memory card. The computer neither knows nor cares what's in that file, it's just a file.
The "something.FIR" file comes packaged either in a ZIP file, or a DMG file. You can get either one, it's your choice. But the ZIP file can generally be unpacked on Windows machines -- any Windows machine -- whereas the DMG file can be unpacked on Macs -- again, basically any Mac.
Either way, you get the package -- the DMG or the ZIP -- unpack it on your computer, and copy the "something.FIR" file to your memory card. The computer doesn't interact with the .FIR file at all, so comptibility isn't an issue. The specific version of Windows or MacOS you're using really shouldn't matter at all. Yes, I know some of the Canon web sites have drop-down menus listing many exact versions of each OS. I believe that this is for software which actually runs on you computer, like Digital Photo Professional. For firmware, it's irrelevant (or at least should be).
So if you have essentially any Windows or Mac computer at home, try this there. If it doesn't work, tell us EXACTLY what goes wrong -- like exact error messages, exactly what software they come from, etc. Oh, and it would help to tell us what camera you have.
