Netscape and Mozilla causing problems (not IE!!!)

I'm trying to make a fluid, rounded design, but I'm running into some strange problems. Please take a look at http://www.jeugdraadoosterzele.be/test/ and resize your window in netscape, mozilla or firefox... you'll see the header shifts to the right (or the menu to the left), so there's sometimes a gap of 1px between menu and the header. Almost the same problem with the windows in the main div.
IE 6.0 (XP), Opera (XP - here there's a problem with the footer), Safari (mac) don't have this problem. (at least not so bad)

I would be very pleased if someone could help me,

ps: (don't mind the double headerbackground, I still have to work on that)