Open this page in Firefox for Android, then press the install button: Install "Show Sync Key" add-on
by Matt Brubeck
The add-on will allow you to send the key to yourself by email. If you press "back" to cancel sending, the add-on will simply display your sync key on the screen.
The add-on also adds a "Show Sync Key" button to the Firefox preferences if you want to send or view your key again.