Firefox 5, which is currently available on the Aurora channel, has received some nice polish that streamlines the primary interface in really nice but subtle ways. Have a look at the comparison:
(For a side by side comparison, rather than stacked, click here)
As you can see, the buttons and textfields are much crisper. The corner radius has been sharpened up a bit and the button and textfield shadows have been tightened up and pulled into the border line.
The overall effect might seem subtle but after using it for a while and then going back to Firefox 4, you can really "feel" the difference.
update: some of you don't see the difference and thanks to Dão's excellent suggestion, I think I've got a way to make it more clear. Have a look at this nifty APNG image :-)
Does that make it more clear? The buttons are more square (the rounded corners are less rounded) and they no longer have the separate drop shadow. The textfield no longer has the shadow inside the top border nor the highlight below the bottom border. The overall look is more streamlined and modern without becoming totally flat and uninteresting.