Google Maps error notifications due to wrong API Key

I use Firefox 13 with Ubuntu 12.04 in two different computer, a desktop and a netbook. In general I have same extensions and preferences in both PC. ''In only one'' of them I cannot display Google maps anymore (the embedded objects in non-Google site), getting this error popoup (ITA): "Questo sito web richiede una chiave API di Google Maps differente. È possibile generare una nuova chiave all'indirizzo [http://code.google.com/apis/maps/signup.html http://code.google.com/apis/maps/signup.html] .". I cannot understand - after googling for a while - if it is a matter of Firefox settings or other OS-related issue (e.g. the java machine).