Fantastic! Rated 5 out of 5 stars
I love this extension and I'm extremely grateful to Kai Liu for creating it.
As others have said, it fixes a somewhat subtle issue, but it does so in a flawless manner and works perfectly.
It may seem trivial to those who don't actually use 3D looking user interfaces, but for those of us on classic looking XP or Windows 2000, it not only significantly improves the look and consistency of menus Firefox / Thunderbird, but it also makes usage more intuitive. It becomes immediately clear that the drop lists on back and forward buttons work separately to the button itself and that these drop lists are clickable. It even has a nice 'sinking in' effect when you press it.
The fact that the author has made this extension Firefox v3 compatible is yet another bonus.