Restore session in Firefox
What does this mean, is it related to the session concept which is stored on the web server?
If I stop the computer with my web-app, and then start it again before the timeout set on the session on the server expires, then anybody that starts the computer and clicks "restore session" could continue in my web-app where I left off.
Is there any way to kill the sessions after I close the browser like in IE and Google Chrome?
You are mixing the "session" as the current identifier of your browser/you in a web application, and the session of the user in her computer, meaning all the tabs opened.
Now for your other questions :
If I stop the computer with my web-app, and then start it again before the timeout set on the session on the server expires, then anybody that starts the computer and clicks "restore session" could continue in my web-app where I left off.
It depends on how you defined the session duration & expiration. You can define the session to expire when the browser is closed. If done so, the user won't be able to restore the session when he'll reopen it's browser. Now if you defined a duration instead, it depends if the session will have been expired or not.
Is there any way to kill the sessions after I close the browser like in IE and Google Chrome?
Like I said, just define no expiration date for the cookie session (if you use it like this) to expire when the browser is closed and it will be good.(see Wikipedia for that.)
I think your question and a solution are described in "Firefox doesn't delete cookies on exit. When will it do?" (https://support.mozilla.org/de/questions/938865).
Also see FireFox's restore previous session restores logins authenticated by server's sessions .
So in short: configure Firefox to delete your cookies on exit and us about:config to set
