Background color trouble FIREFOX

any idea why my menu background is white when I shrink the browser window. As the list drops a line the color finishes on the end of the list item. , it works in IE but not firefox. It works here but I cant fathom it out. I basically want the list to stay grey as it shrinks to one colomn wide, not have white blocks at the end of the lines. come on , its doin mi ed in !

The issue is with #navbackground. It needs to contain the floats, so give it overflow:auto. Then give it the same grey background colour as the UL.