Border disappearing problem with Mozilla

I’m building my new blog with WP, but i found a little problem on the rendering of my layout with Mozilla (Firefox 0.9.1) for Win. Instead no problem with IE 6 and Firefox on Linux.
Using this index there are no problem and it renders so.
As you can see all the 1px blak borders of the sidebar are visible.
But when i add a simple java script on the right sidebar to render a RSS feed something wrong happen: here you got the new index (added of the simple script inside th rightColumns div) and here the screenshot.
The 1 px black left border of the left sidebar disappears for the length of the text there contained and the strange thing is that it reappears or when you scroll down or when you change window and than come back on that window.
( here the CSS used.)
Please, someone help me! 🙂
Thanx