I have ie8 and firefox3.5 and I don't see any horizontal rules at all. I know you stated Safari and chrome and older Firefox versions.
But that is my point with cross browser compatibility.
What browser are you using to see the lines?
What browser do you wish for the lines to work correctly on?
I see items missing using the browsers I use listed above.
Such as the blue arrow for new posts in the left top area shows for Firefox but not for IE8.
Then to the top right where you can navigate with the arrows for older newer topics is missing the explanation text to the left of the arrows on both browsers.