Unfortunately, the Netscape way of doing this is significantly more complex, and is fraught with known bugs and limitations. Essentially, you'll need to create a JavaScript function that you use to make the menu visible/hidden, then call that function wherever you're setting the visibility now. The function will then have to determine which browser is viewing the page, and then use the appropriate code to perform the task.
If this is something you see yourself doing regularly, I'd highly recommend you pick up a copy of "Dynamic HTML: The Definitive Reference" from O'Reilly. It's a great book that provides complete HTML, CSS, DOM, and JavaScript references as well as a 200 or so page discussion (with plenty of sample code) of cross-browser DHTML scripting that will teach you everything you need to produce these kinds of effects in a cross-browser fashion.
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