For a while now, Mozilla has been working on a Labs project called Weave. Weave was an add-on that allowed users to synchronize their customized settings across all of their computers. Google's Chrome browser offers a bookmark and preference sync, similar to Weave. However, Chrome's solution doesn't take its capabilities nearly as far. Mozilla's solution syncs bookmarks, browsing history, preferences, passwords, filled forms, and a nice chunk of previously opened tabs across all of a user's computers. This is done by sending the data, encrypted, over to Mozilla's servers, where it is eventually pushed to each client that your credentials are used on.
According to the official Mozilla Labs site, Weave is finally handing in its training wheels and moving into the spotlight. Now called Firefox Sync, the service has been integrated into the Firefox roadmap and will eventually be hard-coded into a future major release of the world's number two web browser (perhaps it will make it into 4.0). In the meantime, should a user get antsy and want to take advantage of Firefox Sync, it's available as a standalone add-on. The newly re-named project is now available in more than 15 languages, contains a simpler sign-up and setup process, and has a single button to access your remote tabs.
To try Firefox Sync, visit the Add-ons for Firefox site and give it a go.