1. First, download Mozilla Firefox 4 Beta version.
2. Install Firefox and launch it. Click Firefox on the top left corner of the window and from the drop down menu, click Options.
3. In the Options dialog box, click the Sync tab and under that click Firefox Sync.
4. Select ‘I have never used Sync before
5. Now you can create a Firefox Sync account. After you fill up the details, check the ‘I agree…’ term and click Next.
6. Next, enter your Secret Phrase. These will be used to encrypt all your data so that only you can use it.
7. Now you can check what to sync across all the computers and what not to sync.
8. Enter the CAPTCHA code to let Firefox know that you are a human. Thats it! Firefox will now say that it is setup to automatically sync data across the computers.
Article source:http://www.thinktechno.com/2010/08/25/mozilla-firefox-4-how-to-use-firefox-sync-feature/