"Firefox 4′s brand new Sync feature is one of the main
attractions of the latest release. It allows you to sync browser
data, such as bookmarks, preferences, history, and tabs – but
not extensions – across multiple Firefox installations.
Here's how it's done...
"01 Setting up Sync Setting up the Sync feature from scratch is
a cinch. Choose Tools>Set Up Sync, press the 'Create a New
Account' button and fill in the required fields."