One of the main things I missed from Firefox was the little warning that popped up when closing a window if you had more than one tab open asking if you want to save the tabs or quit entirely.

I found two so far that come REALLY close;

Tab Saver: The fact you have to click the icon every time you open/close chrome isn't terrible, just awkward.

Last Time: It automatically saves your tabs, regardless of whether or not you want it to. Which I find better than the last, but it would be PERFECT if it had a little pop-up confirmation window when you have more than one tab open (save/quit/cancel).

Love the increasing quality of the extensions though.