What are the prerequisites at the computer user to use Alfresco Cloud (browser version, …)? Can you confirm that the version 3.5 of Firefox causes problems at the file upload? Same for IE6.
Alfresco Cloud supports Internet Explorer 7+; Firefox 6+; and Chrome 15+
Since Alfresco Cloud & Enterprise are branches of the latest Community release, you can look at the official Enterprise supported platforms, they apply to Community and Cloud: