just been onto support through chat. i posted the forum link to show pics but dont think he looked as kept asking to try chrome instead of firefox, that was after the oh we have special offers for you of vpn and cleanup yada yada.
he then said what makes you think it is avast causing the issue. so i said you did read above where i said it worked fine all morning until i updated then rebooted and issue since.
so i tried chrome, had to wait once again for two factor code to other email and logged in but of course as i had the chat tab open to avast the email stayed logged in. i then tried closing the msn and yahoo email tabs and turned avast off. logged in again and closed.
then opened again and low and behold the emails stayed logged in. turned avast back on, closed emails and tried again and logged out. in the end had to end the chat to check if i was logged out of all emails and i was.
so in simple terms, turning avast off altogether solves the issue and emails stay logged in. so next will turn off the brouser data protection only and try all the emails again.