In Firefox 4, script execution changed to be more HTML5-compliant than before. This means that in some cases sites that sniff for Firefox or Gecko may break. If you use LABjs… You should update to LABjs 1.0.4 (or later). If you use the “order” plug-in for RequireJS… You should update to RequireJS 0.15.0 (or later). If you use the multi-file version OpenLayers… You should switch to the single-file