Sync lets you synchronize your browsing data such as bookmarks, history and passwords between installations of SeaMonkey or Firefox (e.g. your computers at home and at work) or mobile phones that support Firefox Sync.
All data is stored securely in a way that allows only you to access it (encryption happens on the client side). Mozilla provides a default Sync server free of charge, but individuals can run their own, too.
Sync uses a set of credentials: A user name (email address), a password, and a Sync Key. The user name and password are used for communication between clients (browsers using a certain Sync account) and the Sync server, i.e. the password protects you from others that might try to take over your account. The Sync Key on the other hand is used to encrypt all data before it is sent to the Sync server. Unlike the user name and password, which are known to the Sync server, it is only known to you.
After the initial setup, Sync operates in the background, continually and automatically synchronizing your data with the Sync server (manually triggering a synchronization is possible, too). Clients that run simultaneously and access the same Sync account have their data merged on the fly.
The following assumes that you already set up Sync on at least one device (see steps above) and have both devices at hand.
Alternatively, if you only have your second device at hand, you can set it up using the Sync account information (email address, password and Sync Key / Recovery Key). Following are the steps you need to take in order to set up another instance of SeaMonkey. For devices running Firefox, see the How do I add a device to Firefox Sync article on Firefox Help.
To determine your Sync account information, follow these steps: