One fix that I really like is the one related to moving focus after a tab is closed. Right now, if a link opens in a new tab and gains focus, and you close that new tab, you are taken to the right-most tab you have opened. The fix makes it much friendlier by taking you back to the "parent" tab that opened that link. Hurray!
Another change is the addition of big "X"s (crosses) on the right of each tab which you can click to close tabs. Something like this:
Well, rather nice for people with only 2-button mice or using specific OS ports of Firefox (I'm thinking some flavors Linux desktop environments, but I'm not sure), since they can now close tabs in a single-click (instead of right-click -> Close Tab). Personally, I prefer to middle-click tabs to close them or hit Ctrl-W, and the additional close widgets are just unnecessary clutter (so it's a good thing this can be turned off via about:config at the moment). Anyway, if you're using a 3-button mouse and are closing tabs with right-click tab -> Close Tab, stop doing that! Middle-click the tab. Middle-clicking is (or should be) the new "close me" in any environment that involves tabs - think IntelliJ IDEA and editors like PSPad. If we could just get other applications to follow this "new convention" (think Eclipse IDE, EditPlus).