That's because Enhanced Tracking Protection clears cookies, which will log you out of most sites after you close the app. To get around this, change your cookies settings so that they don't block ALL cookies, or list some of the sites you want to stay logged into as exceptions. Also, make sure you haven't set your browser to clear cookies after each session

Hello,

       This problem is fixed with latest updates... Previously, I was still trying with different methods to get it solved as...

1. Disabled all add-ons, but doesn't solved that... 2. Reinstalled Firefox after clearing data... Didn't worked... 3.Totally Disabled Enhanced Tracking Protection... It worked temperorily, as I again turned it on with "Standard" protection, not "Strict"... It happened again & get log out of the sites...

Thank you for letting us know.