Navigation links breaking onto next line

The top line of links on my website looks great in the latest version of Firefox, but in most other browsers the last link breaks onto the next line. I had fixed this problem, I thought, by using ul and li tags, but when I had my hosting company copy my design to another website of mine, suddenly the problem resurfaced.

Here’s my website: LINK

Can someone please take a look at this and help me figure out why the links are breaking onto the next line? I’ve already validated my code and only have two warnings from my Twitter widget. Otherwise things are in great shape.

Many thanks!!