In typical web design fashion, there's always something that doesn't look quite right between two browsers. This one just completely lost me though.
I open the site in Safari. Everything is dandy.
I open it in Firefox. The entire page is displayed, just fine, but about 10-20% smaller. Images, fonts, divs. Everything.
Does this make any sense to anyone? Have you seen it or fixed it before? Is it just this browser? Is it me? Have I finally lost my mind?
You may have zoomed firefox's view by accident, ctrl-+ will increase the size, ctrl-- will decrease the size. To reset it, hit ctrl-0.
As Brian has commented on this answer, you certainly shouldn't rely on the size being the same, though I'm sure you're aware of the perils of designing for a particular size. It's good to know that you are seeing what you intended though.
You should never expect a website to look exactly the same, cross browser, cross OS, etc. etc. Some people will have smaller or larger resolutions, others will have text size zoomed in or out. Therefore you should always keep these things in mind when designing your website! Don't design for pixel, but for more fluid layouts.