I have Firefox set to delete cookies and site data upon exit. I have never had a problem logging in, and once I do, I'm logged in all day. All I can think of is under Tools / Settings / Privacy and Security / Cookies and Site Data / Manage Exceptions, you have https://www.coincommunity.com in there set to Block or Allow for Session. I can't explain why it would work for a while and then not, if you are not altering this setting.

