Thanks for including the screenshot; that makes it a bit easier to troubleshoot. 👍
It appears the site's custom font did not load. The browser fell back to Verdana, which is a bit too wide for the fixed-width template, and it broke the header. The question, then, is why didn't the font load? It looks normal for me in the latest version of Firefox, and I don't see any problems with the font in other browsers, so there's not much else I can do.
My first advice would be to clear the cache. That should force the browser to re-load the font file from the server.