Hi, Mayan, and welcome to umbraco!
One of the great things about umbraco is that it doesn't "get in your way" and change the xHTML/CSS output. What you tell umbraco to send to the browser is exactly what goes to the browser. So if there is a display issue in a browser, it probably isn't umbraco but your xHTML/CSS.
The first thing I'd check is to be sure your page is xHTML and CSS compliant.
Go to
http://validator.w3.org/ and
http://jigsaw.w3.org/css-validator/.
If the page validates then it maybe a bug in FF3's beta.
Let us know what you find out.
cheers,
doug.
MVP 2007-2009 - Official Umbraco Trainer for North America -
Percipient Studios