There are several excellent bookmark-syncing extensions for Firefox, but Weave is the first one designed by Mozilla. As you might expect from the company that produces Firefox itself, the appeal of Weave is that it synchronizes more than just bookmarks, and also supports syncing open tabs, history, passwords, preferences, and Firefox 3's advanced Location Bar data.
The add-on lives in your Tools menu and in your Options menu. From there you can check your syncing log, create a user account and password, and toggle sync settings. The default is to sync all. But in practice, Weave has had a rough development history, and the road ahead hasn't yet been smoothed out. All the features that it is supposed to sync worked in our tests except for tabs, and the add-on still has compatibility problems with certain high-profile security add-ons such as AdBlock Plus and NoScript. These are easy to disable and then re-enable to get a one-time sync of key data such as passwords, but prevent Weave from being a reliable tool. Also note that the tab sync is only supposed to work with 25 or fewer active tabs.
Mozilla says that future versions will sync add-ons, personal tweaks including any changes made to the about:config, and across mobile devices. Weave will be a great tool for Firefox once it's working flawlessly, but it's not there yet.