When you say this page, are you referrning to Stackoverflow.com? SO works great in all the browsers I've tried it with.
If you are referring to your own site, you need to tell us specifically what is going wrong or at least give us a link so we can see the errors you are getting.
I have no idea what language it's in, but it's displaying fine in Chrome - which is not a Firefox-like browser :)
At the very least you are going to have to deal with the fact that the CSS :hover is not supported in IE for any tag except the anchor tag. You'll need to use onmouseover, onmouseout in IE to accomplish the same thing. Or change the li:hover to an a:hover but that would be a bigger overhaul of your design I think.
Specifically, CSS like this:
ul.menu li:hover ul.submenu {
background:white none repeat scroll 0 0;
border:1px solid #A6A6A6;
display:block;
margin-left:-25px;
margin-top:23px;
padding:2px 0;
position:absolute;
}
Is not going to work properly in Internet Explorer. I don't know about Opera.
I had this issue with a site I created. The problem ended up being that I shouldn't have ended the <li> tag before the next <ul> tag started in the menu.
For example:
<div id="menu">
<ul><a href="/index.html">Home</a>
<li><a href="/aboutus/index.html">About Us</a>
<ul>
<li><a href="/aboutus/history.html">History</a>
Note no closing </li> tag.
I ran into a lot of problems with IE and CSS Menus. I recommend the following resources, they may save you a lot of time:
Let me know how that goes.
For what it's worth, the first resource includes a csshover.htc file and its usage -- which the site I created uses to fix the issues caused with Internet Explorer. If you view the source for the site, you'll also see conditional comments that pertain to Internet Explorer.
Without seeing the CSS for your site, I suspect that you are using css:hover with Lists to create the menu. IE doesn't support this, so you have to supply a behaviour file to allow IE to have the functionality, Link with a file here
If you're familiar with javascript, you might want to look into the JQuery Superfish plugin. (http://plugins.jquery.com/project/Superfish). Most of the hard work and weird browser issues are already taken care of for you.