I think this has more to do with Browser and Online security to insure the user logging in is whom they say they are. The analogy goes like this would you like to keep your car unlock or home unlock because you come and go everyday? It's most likely go to say "No" right. They have security to protect and if hackers gain access to a unrestricted system it become a "BotNet" or "Zombie Network" and we seen and read enough news of such attacks and "Ransomware" to know it's not safe to leave system open to anyone or anyone "online". So those connections must be made again with correct login/pswd to insure only the user has access to them and not some malware access. And this logout when tabs or Browser closes isn't only to Firefox but Chrome and Edge when I use them even when their password remember is active I still have to choose the correct login for the site but I also Password Protect my "Admin" account so nothing can just install Malware as I only use the "Limited" account that if it wants to install anything the Admin password must give permission and if your not installing then that's a "RedFlag" to ask what is going on here. So it's safe to say the Browser or tab kicking out is for a "Good" reason and yes it does give headache but for "Good Reasons".