Well ... you have some kind of in-browser font-zooming going on in that browser. I can see that because your fonts are quite a bit bigger than default
download/file.php?id=547 ... Besides, its not just breaking by a single menu element, but two ... These things you will have to take into account. It's not like your menu doesn't wrap on smaller screens, although it fits to pixel-perfection on my 1280px macbook. One more letter, and it would wrap. I can't "fix" this ...