While i'm typing in the URL, the letters i type are antialiased at the subpixel level. In the enlarged picture, you can see colour fringes around the letters — Mac OS is using those colour fringes to make the letters look crisper and smoother.
But as soon as i hit Return, the URL is redrawn with antialiasing in whole pixels. Nothing but grey pixels here.
The URL stays that way while Firefox is trying to connect to the remote webserver. Once the connection is established, the URL is redrawn with subpixel antialiasing again.
In fact, you can test this out without having to watch for a brief split-second flash of whole-pixel-antialiased text. Just type in a URL with a domain that doesn't exist, like http://xyzzy.google.com/, and after the connection fails you'll be left with a whole-pixel-antialiased URL. Compare how the "google.com" looks in "http://xyzzy.google.com/" to "http://www.google.com/" to see the difference.
I wonder if anyone else has ever noticed this.
(By the way, if you are one of the few people on this planet crazy enough to find this kind of attention to detail somehow endearing, i want to know. And if you also happen to be female, i want your number.)