The "Secret Phrase" is a passphrase created when you set up your Firefox Sync account. It is used on your computer to encrypt your private data. To protect your privacy, the secret phrase is never sent to the Firefox Sync server. In some recent versions of Firefox Sync, the "Secret Phrase" is now called the "Sync Key." (Same thing, only the name has changed.) If you have not set up a Firefox Sync account and secret phrase, follow the instructions at this web site: http://www.mozilla.com/firefox/sync/