If it happens in Firefox and not in Chrome (did you already try Internet Explorer?) it's unlikely to be a Windows 7 problem. That's easy to confirm by running Firefox from a Ubuntu disk.
- What's the resolution of your monitor (like 800x600, 1920x1080?).
- Do you have a link, so I can look myself?
Different browsers can 'render' a html-page in different ways. That has nothing to do with your OS, but with the way the page is coded on one side and your display on the other side.
If such happens, and we can't change the site, we use another browser. And you might contact the webmaster of the site to complain about it.
But first, I'd like to have look myself.