I start up my browser on an empty cache, no passwords and no nothing stored. Log in to Stackexchange via the Google log-in option.
Next, I open feedburner.com in a second tab. (I have never had an account with feedburner.) The page greets me, "Welcome back, [e-mail address]"!
How is that possible? Apparently the log-in to SE created some kind of super cookie in my Firefox that any website that gets opened in a new tab can access. I had no idea that could happen. There are web sites that I visit that I do not want them to know my e-mail address.
If, that is, it is only the address and not also the password!