I've been using Firefox for years now and have never had a problem downloading any Calibre update.
I do go through Calibre...when I open Calibre it notifies me of an update...I click to update and Firefox opens to the Calibre site and I download and install. Easy peasy.
Thank you for the most recent offering of Calibre. I've used it for quite some time on a variety and have had very few, most of my own making, problems with the program.