I bought The Witcher 3 GOTY and am presently trying to download those 3.9 GB bin files. I'm downloading through Win 10 + Firefox. My problem: sometimes the download breaks (failed) and the retry button doesn't reconnect and resume the download where it broke. I have to start all over. And, for me, it takes one whole day to download a 3.9 GB file.

How can I improve the reliability of my downloading, so that I can avoid wasting day after day restarting a broken download from scratch?

I'm still using GOG downloader, you can get it from this url https://www.gog.com/downloader no issues to this day.