One thing I've been waiting for is for one of the tabbed browser creators to recognize that if I have a window full of five or six tabs, all active, and I close the window when I really intended only to close the currently visible tab, I get really annoyed with myself!

A confirmer for a window-close event if there are multiple tabs seems like it shouldn't be too hard.

And a tip. You can save a collection of tabbed windows as a single bookmark. Very handy. I have three sets of them for different roles I play on the Web.