If you use both the Undo Tab feature and Panorama (aka “Tab Candy”), you might want to hold off on Firefox 4 beta 9.

Bug 624265 (“Undo most recently closed tab creates a new tab group and switches to it”) is a bit of a nuisance, and though it’s since been fixed in the nightly builds, it wasn’t fixed in time for the beta release. (I use the nightlies, myself.)