In a recent update (v70.0), Firefox updated their security and privacy functionality. The new update is much more strict on stored cookies and tracking protection. Therefore the cookies are not accessible when you re-open the website and it requires you to log in again.
IMPORTANT: Always follow the security guidelines of your organization! Check the suggested solution below with your IT or Security team before implementing it.
If you are struggling with this issue, change the Privacy & Security settings as shown in the screenshot and you should be fine.
1. Type the following to Address bar and press Enter:
2. Under Enhanced Tracking Protection, choose Custom.
3. Tick Cookies and set the value to Cookies from unvisited websites.