Hi Hugh,
From the information you provide, I know that you can use the services (Exchange Online, SharePoint Online and so on) in Office 365 successfully.
So what step did you do when you encountered the error message “Firefox can't find the server at www.giant-move.us”. From the error message, I think maybe the error appears in Firefox. Does the error message appear when you login to Office 365 portal or OWA(Outlook Web App)? Please provide more information about the issue for us to troubleshoot it.
Additionally, please post a full-screen shoot about the error message here for us to search.
Thanks
Linda Wang