If your Firefox has already stored your log in information for sites like Gmail, Facebook,etc. then you can use private browsing so that you will be logged out automatically after you close Firefox. To enable private browsing press CTRL+SHIFT+P.

I haven't saved any passwords and sometimes when I exit I haven't finished my work. I tried Private browsing and it exited all my tabs and didn't prompt me if I wanted to save my tabs. I want to save my tabs that I'm using for the next time that I enter Firefox, but I want it to log out of my email and Facebook accounts in case I forget. Even though this is a personal computer sometimes other family members do use it.

You can adopt the following methods;

You don't need to be in private browsing mode.

about:config

browser.sessionstore.privacy_level set it's value to 1

{It may not work for all HTTPS sites but I have tested it on facebook,gmail and it worked.}

signon.autofillForms {double click it to set it's value false}

{Prevents auto fill up of forms.Firefox won't autofill the name and password. You need to open the name list with a double click in the name empty field.}

browser.tabs.warnOnClose {Make sure it's boolean value is true.}

about:sessionrestore {For session restore.}