Viewing your homepage http://www.xolox.nl/ in Mozilla 0.9.3 gives the user undesired results. It is impossible to view anything except the home page due to the menu not operating properly. Some research would have given you the script http://www.simplythebest.net/info/dhtmlscript79.html , from the same site you got the menu script you are currently using. The script listed here is cross-browser. There are other scripts available on that site, that's just the first one I saw. This is not a Mozilla bug.

The table on http://www.xolox.nl/text/welcome.html also appears incorrectly, but looking at your code I am unable to determine whether this is a coding error or a bug in Mozilla.

A quick test also revealed that your center frame is too wide, being unable to be viewed on a screen set to 640x480, regardless of browser.

Nitpick: the black used in the background of your images is not the same as the black used by either IE 5 or Mozilla for the background.