Google Browser Sync is a Firefox extension that lets you synchronize your bookmarks, history, persistent cookies, tabs, windows, and saved passwords across your computers. It's useful if you a have a desktop and a notebook and you want a coherent browsing experience.
During setup, you need to provide a PIN to encrypt information that's synchronized between your computers. That means the information will be transmitted to and stored on Google's servers in a format that can't be decrypted without the PIN.
Another extension that has a similar purpose is Firefox Extension Backup (FEBE), but it can also backup extensions and themes.
{ Via Google Official Blog. }