Google Browser Sync is a new plug-in for Firefox that will allow you to keep multiple copies of Firefox (home, work computers?) in sync. Your bookmarks, history and cookies are all continually synced. Very cool — If you don’t mind google having a running record of everything you do online.
Update: I played with it a little bit, hoping that maybe it would provide a nice way for a person to log into any installation of Firefox and make it theirs. While it is conceivable that you could use it in this way, it really nags you to enter a login and password and not everybody would use it. It seems best suited to being used by a single person (like the home/work example earlier).