There are several excellent bookmark-syncing extensions for Firefox, but Weave is the first major effort designed by Mozilla. The newest version of Weave is out of beta and supports more than just bookmark sync. It also supports syncing open tabs, history, passwords, preferences, and Firefox 3's advanced Location Bar data.
The add-on lives in your tools menu, in your Options menu, and in a Status Bar icon. From clicking on any of them, you can check your syncing log, create a user account and password, and toggle sync settings. The default is to sync all, but choosing the customization option will allow you to toggle options.
The appeal of Weave is that it's designed to be a one-stop syncing shop. In practice, though, 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. To its credit, Weave tested successfully on both Windows and Mac.
Mozilla says that future versions will sync add-ons, personal tweaks including any changes made to the about:config, and across mobile devices. It currently works on mobile Firefox for Nokia's Maemo platform. Weave will be a great tool for Firefox once it's working flawlessly, but it's not there yet.
If you've had a different experience with Weave, let me know in the comments below.