Here are the steps “how to retrieve Firefox Sync Data”
• Installed Firefox 4 Beta
• Setup Sync.
• Firefox synced all my bookmarks correctly.
• Re-installed Windows re-installed Firefox 4 Beta.
• When setting up Sync, I chose "Merge Data" by default, instead of choosing "Get data from server".
• Now my Firefox, even after Syncing, doesn't show any of my old sync data.