I am using a table to render a calendar. I have noticed an odd rendering bug in table cell border rendering in Firefox version 3.6 through 7. Here’s a screen capture:

As you can see, the border gets “bent” when I scroll. Also, there are gaps between the horizontal and vertical border where it isn’t “bent”. A live example can be seen on this web site.
I don’t see this behavior in Chrome, Safari, or Internet Explorer.
UPDATE
I am still seeing this issue in Firefox 20. I have noticed that single pixel borders do not exhibit this behavior, only two pixels or more.
You have a problem whith border-collapse:
Here is a solution for your problem: http://www.charlesgarwood.com/blog/?p=13
What to do:
border-collapsefromcollapsetoseparateborder-widthof the<td>s and<th>s from2pxto1px<table>itself a1pxborderGive some conditional comments like described in the link