When you switch to one of the other tabs, that's the first time they are actually instantiated and run. So if you had this tab in the background, your machine crashed, and Firefox reopened on boot, it would not crash again.

Often, it will report "We are having trouble restoring your session", seemingly without reason, and that dialog has a very simple system for choosing which windows and tabs to recreate vs which to discard. I think you can change a setting to always use this dialog.

I can't recall if chrome has the same interface.