One feature I missed very much on Firefox was the option to easily and automatically save browsing sessions. Opera had this feature even in the old days, out-of-box.
[:http://tmp.garyr.net/|Tab Mix Plus] - an extension to Firefox adds the feature in a pretty nice way. It saves all the tabs even when the browser crashes and more importantly, lets the user open back the tab closed accidentally. Another cannot-do-without extension for Firefox. Hope features like this one get added to the core distribution.