There are a few factors you have to take into consideration when troubleshooting this issue:
1. CSS (as XHTML) is case senstitive, and FireFox has much more respect to this than IE, that is why HTML != html.
2. Take a look at the doctype of your file - if it is XHTML 1.0 or 1.1 the overflow property of CSS is applied to the <html> element, not to the <body>. This is also valid for the scripts that manipulate the DOM.
These two factors may be the cause for FireFox not working properly. As I do no have your files, and only the CSS, these are the only guesses I can do about the problem.
Have a great weekend,
the Telerik team
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