Same person, different journal. Okay, so I've tested it specifically in a layout coded by a user, and then inputting custom styles just to change font on one of the system's preloaded S2 layouts.
To do it in a layout coded by a user, you need to put the code from that site above into the very top of the custom stylesheet box on the custom options page, and then go find the section of CSS where it styles the part of the page where you want the font to appear. Then, you'd just insert the font name as though it were a regular web-safe font, with quotes if it's got a space in it.
To do it in a preloaded system layout, you need to put the code from that site above into the custom stylesheet box, and then, find out what the specific element you want to edit is. (I'd recommend using something like Firebug to specifically inspect the element.) Then, using that element name, put in something like this:
element {
font-family:"Font Name", georgia, serif;
}
(For the preloaded layout on this journal, I targeted .entry-body to change the entry text. The specific element changes from S2 layout to S2 layout.)
I'm not seeing any layout breakage in Firefox, but if the problem continues, it may be a browser compatibility issue (not all that likely, but not totally impossible) or another problem elsewhere in the code.