Firefox Sync, originally branded Mozilla Weave, is a browser synchronization feature that allows users to partially synchronize bookmarks, browsing history, preferences, passwords, filled forms, add-ons and the last 25 opened tabs across multiple computers.
It keeps user data on Mozilla servers, but the data is encrypted in such a way that no third party, not even Mozilla, can access user information. It is also possible for the user to host their own Firefox Sync servers, or indeed, for any entity to do so.
Firefox Sync was originally an add-on for Mozilla Firefox 3.x and SeaMonkey 2.0, but it has been a built-in feature since Firefox 4.0 and SeaMonkey 2.1.
Read more about Firefox Sync: Firefox Home, Sync Server, See Also