You need to sign in to your sync account in Firefox on your new PC before you can use it. To do so, follow the instructions in this article: Connect another computer to Firefox Sync

KuchleiT said

I have an old pc running windows XP (Up to Date) and have just purchased a new one running windows 7 I am trying to transfer all FF data from from the old to the new. Tried sync but keep getting an error, "Sync encountered an error while syncing: Unknown Error. Sync will automatically retry this action." Obviously I am doing something wrong but don't know what.. Can someone please provide a step by step instruction as to how to accomplish this. Thanks

That did the trick. Thanks very much for your assistance.