hi, a master password protects access to somebody going into the firefox preferences > security settings and taking a look at all your passwords or once credentials are about to be autofilled into a login form.

you staying logged in at a website through multiple sessions requires neither of those possibilities - this is controlled by cookies that websites store in your firefox profile. so you should either untick the option to stay logged in during signin to gmail or set firefox to clear cookies after the session...

Enhanced Tracking Protection in Firefox for desktop

So I changed my settings. Under privacy, History I now have selected "use custom settings for history" and there are checkmarks for "remember my browsing and download history" "remember seach and form history" and "accept cookies from sites. Accept third-party cookies is set to "never" and Keep until: "I close Firefox".

still aftor doing that closing firefox and starting it again, I'm automatically logged in to gmail. Even after reboot, firefox will log me in into gmail.

that setting will only apply to new cookies - so please delete all your existing cookies from google.com.

i'm also not sure if this setting will be overwritten by session restore which might also keep a record of the cookies of the sites that where open when you closed the browser - more infos and workaround here: https://support.mozilla.org/en-US/questions/970539#answer-476964

Thanks Phillipp!!