Recover Sessions - Firefox

Today for whatever reason - my FF session crashed and it crashed big time. I had about 10 tabs open. When I reopened FF, it asked me if I wanted to restore my crashed sessions. Wow - that is something nice to have. I wish IE7 puts that in their official release. Maybe it is some add-on I added or does it come with by default with the FF version available for download? I don't know, but anyway its a nice feature to have.