It looks like the problem is that when the web page expands the table cell that contains the logo (about line 74 in homex.html) is expanding lengthwise, pushing the menu down. In IE that cell is only big enough for the logo, but in Firefox it occupies about 60% of the page height.
Move the table with the menu (starting about line 124) and move it to the same cell as the logo. This works in both IE and Firefox (I'm at work right now and can't test in Safari). If you need extra padding between the menu and the logo, you can insert a div with a height style.