I've never seen that before. Are you using Firefox version 32.0.3? Do you have any Firefox add ons or extensions installed? I assume this doesn't happen in IE or Chrome?
I've got it now as well. It seems like the latest version of Firefox is not rendering the page properly at certain zoom levels. Until a fix is in place you can turn the zoom up or down a notch within Firefox (ctrl and scroll wheel up or down) or use a different browser.