One of the problems with running multiple computers is syncing your browser's bookmarks, passwords, preferences, history, and tabs. Well, Firefox 4 comes with an add-on that allows you to sync this information across multiple devices (running Firefox!).
It's more convenient than backing up bookmarks because the service allows you merge data on different computers. Moreover, the data is encrypted on your PC before it's uploaded to the server, so data such as passwords should be safe.
Firefox Sync works with Firefox 3.5 and 4.0.
Sources & References
Mozilla Firefox Sync 1.7