I think GoDaddy is right, its unlikely the new owner of the domain name uses your old homepage, why not purge your browser cache and then visit the old site again and lets see what happen.
You can also get a friend to look at the domain name (from a different computer) and tell you what he can see.

i use godaddy for some of my sites, i would imagine its just a catched page your seeing or maybe the new owner has retrieved your page from the internet achieve to put on his newly aquired domain, this way if he can replicate your site and slowly change the site in his own time, it will maintain pagerank and serps positions that your old domain had.

Dont take this as gospel, it just what happens sometimes and ive even done it my self once to maintain excising Serps rankings, with out looking at the new and old site i couldnt tell you for sure whats going on , feel free to PM the address and ill take a quick look

So you have definitely cleared you cache in Firefox/tried in a different browser?

Get GoDaddy to close their account for breach of Ts + Cs if they are stealing your website they are breaking the law

are you using wordpress? have you changed the config to reflect the new Db details?