There is a dark bar being drawn between columns at:
This is not visible in Fx.
Note that the browser window must be made quite wide until two columns appear and the bug is exhibited.
Can someone check whether this issue exists in other ports (eg. Mac)?
Created attachment 16788 [details]
Dark bar between columns
The CSS has:
-webkit-column-rule: medium solid;
which suggests that this is intentional. I guess Firefox just doesn't support this wonderful bit of work.