I had a look at your WB Karen and first of all let me tell you that is some top notch work! It appears though that the issue is with Firefox itself on Windows 7 and not with the WB, as it does it in just about any WB I apply and the background color of the line is 151,151,151 in all cases. If you look closely too you will notice that the line is also on the inside of the left frame and at the top of the bottom frame. So there is apparently nothing you can do. I also opened IE9 while I was at it, and it is even worse with a 2 pixel line of different colors in those same areas. Apparently it is something to do with the browsers themselves though for the most part. To confirm things and just for the heck of it I loaded your skin in Windows XP, and the issue did not exist in Firefox, but it did in IE8.
Browser and/or OS issues are the culprits! Don't you just love skinning!
Sorry I could not resolve it for you.