OK, I got a clarification from Venom, our main webdev...

TL;DR: clear Firefox's history and it will work.

We had to change our network infrastructure so that now https://www.gog.com no longer works (and it's impossible to implement a redirect or custom error message). This much you already know, and here's where Firefox makes it interesting.

Because Firefox wants what is best for you :P it will always choose a HTTPS connection if possible. It determines the "if possible" part by checking your history. If you type in "gog.com" in the all-knowing address/search magic bar, and you visited https://www.gog.com/ at some point in the past, it will _always_ choose the https:// address for you, which in turn leads you to an error page.

Long story short, you need to either clear your browsing history (and from personal experience I know this does not always work in Firefox), or manually remove all entries starting with https://www.gog.com/ . I will see if I can contact the Firefox folk and check if they can fix it on your end.