Mozilla errors when using PageMethods.xxx

I have a problem that only occurs at Mozilla, not in IE, Opera or Chrome.
Anyone have a clue?
 
Firebug
onGetNrPagesComplete is not defined
getMapObjects()DesktopView.aspx (row 208)
findMapObjects()DesktopView.aspx (row 247)
function onclick(event) { findMapObjects(); }(click clientX=408, clientY=636)60 (row 2)
[Break on this error] PageMethods.GetNrOfPages..., onGetNrPagesFailed); \r\n
  JavaScript // call to Web Method GetNrOfPages on server to initiate paging settings
PageMethods.GetNrOfPages(selectedCategories, showOption, onGetNrPagesComplete, onGetNrPagesFailed);                                
              &n ...