Details like websites remembering you (log you in automatically) are stored in a cookie. You can remove the Gmail (Google) cookies to make the website forget about you.

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

Note that the Gmail sign page has a button to remember you (Stay signed in), so make sure that this choice doesn't have a tick.

Type about:permissions<enter> in the address bar. In the search bar, enter the name of the web site. Click on that site. At the top right side of the page, find Passwords.