ok, I found parts of a possible solution:
<asp:Label [....] onMouseOver
="this.click();" onMouseOut="AjaxControlToolkit.PopupControlBehavior.__VisiblePopup.hidePopup(); return false;"></asp:Label>
This works fine in IE, but FF does ignore the event "this.click();" and just brings back the errormessage "this.click() is not a function"
Any ideas?
B.