I've spent hours accross the recent years - and only found some awful temp solutions - to prevent Youtube and Gsearch to log me in but being still able to access gmail.
And for God's sake, it's working like a dream come true!!! With the addition of some scripts in Greasemonkey, I can safely and comfortably browse Youtube again! Thank You !

...Now I only need to find a module to hide the damned "Firefox VPN" invasive commercial spread everywhere in this addon...