Have you ever seen this window?

It’s one of my favorite Firefox dialogs: it lets me restore all my windows and tabs when Firefox starts. Like most users, I usually have a constant set of certain windows (emails, blogs and a few other informational sites that I consult throughout the days) along with incidental windows depending on where my web surfing takes me.

There is one tiny problem with this dialog, though: I just don’t know how to configure Firefox to display it.

The only thing that seems to work is to kill the Firefox process.

Does anyone know a way to configure Firefox so that my windows and tabs are always saved when I exit?

Update: Thanks to Rob for the answer in the comments. The setting is hidden in the about:config panel.