See on the top and bottom?  It has never looked right to me.  So I did a little playing around with the Stylish extension for Firefox, and turned it into this:

Now I am far from being a Jedi master of Stylish, and writing user styles.  I am just learning the basics.  Since this was a big annoyance to me though, I thought it might help somebody else out there.

Download Stylish, and then go down to the task bar icon for it, right-click and select “Write Style” and then “Blank Style”.  Type in any description you wish (mine is “Round Address and Search Boxes”) then type this into the text box for your user style:

#urlbar {
-moz-appearance: none !important;
-moz-border-radius: 3px !important;
padding-right: 1px !important; }

#searchbar .searchbar-textbox {
-moz-appearance: none !important;
-moz-border-radius: 3px !important; }

Hit preview to test it out, and if it worked – then hit the save button.  This worked for me using Firefox 3/Vista Ultimate, so hopefully it will work for you.  If anybody has any pointers, be sure to share!