When you create a library using the Zotero Firefox extension, data is saved to your own computer. Your library will not appear on zotero.org unless you sync your library with the Zotero server. To view your local library, click the Zotero icon in the Firefox status bar or select Zotero from the Firefox Tools menu.

If you have enabled syncing and performed a sync and still don't see your library on zotero.org, changes may not be syncing.