I have this problem as well.
But I believe they are trying to encourage us to buy the paid version, so we can create multiple emails in the same account..
I have many email accounts as well, and this is a problem. But maybe with the paid version this problem would not exist (I never tried the paid version so I don't know)
You could even set up multiple Firefox profiles for different accounts.
Proton mail has an auto log out feature that cannot be turned off.so after a few minutes of inactivity or if you close the tab, you are automatically logged off.
This is for our security so I doubt it will ever change...