Float in Firefox

chi mi puo spiegare come mai il "float" si comporta in modo diverso tra IE7 e Firefox ?
vi allego un esempio elementare http://www.itinerarialpinistici.com/test/prova.htm

Div vanishes when using IE6, please help.

First of all, I would like to introduce myself. This is my first post here as I'm fairly new to complete CSS layouts. My name is Tim and I'm from the Twin Cities area of Minnesota. I've done a few sites with strictly CSS and never had a problem but this one is driving me nuts. I'm having trouble with my Navigation ul completely vanishing in IE6 at random times. The site I'm referring to is currently live at DigitalTown.com if you would like to check it out. Here is the code.

HTML

         <ul id="topnav">

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