michal.czernow: I easily recreated this bug, by setting some cookies accessibility options, namely I ordered FF to ask me every time, when a domain wants to store a cookie. I needed to mark to remember my choice for .gog.com domain. Hope that will help.
HypersomniacLive: Are you telling me that the problem lies with me clearing my cookies every time I close my browser?
if yes, this begs the question, why would GOG properly work in v.42.0, like it has in previous versions, only if I keep my GOG cookies?
Did I misunderstand you?
Well, think of this (it may or may not apply). We have a JavaScript application which reads from a user's local storage. If the item doesn't exist, the recommended plan of action is to create an object with appropriate defaults and save it. However, after so much development time, some developers forget to set appropriate defaults if they don't exist in certain areas of the application and it is prone to error.
I can usually detect this by deleting my cookies and local storage, then visiting the site as well as looking at any responses from Fiddler or any errors using the Developer Tools in Chrome. I usually find the problem fairly quickly from the client side, but that's because I know where and how to look.
Having worked on server code, I know that there are exceptions that happen on the server side or perhaps "garbage" returned back by service calls that may throw exceptions, but get gobbled up remotely.
Clearing your cookies or local storage may or may not affect the application. However, if the application relies on information from a cookie and doesn't validate the result, it could cause issues.