Users need to create an account and authorize it on all computer systems (or Nokia N900 handsets running Fennec) so that the data can be transferred from Mozilla's servers to each device. If you'd rather sync up your data to your own hardware or cloud space, Mozilla also offers instructions on setting up your own server. Weave is open source and licensed, like Firefox, under the GPL/MPL/LGPL tri-license.
Currently another add-on called Xmarks does bookmark and password synchronization too, and across other browsers, but Weave should provide a more comprehensive solution for Firefox users specifically. Future versions of the service should synchronize add-ons, personas, search plug-ins, and even under-the-hood customizations like about:config tweaks.