http://lifehacker.com/396112/google-browser-sync-discontinued-no-firefox-3-support
Googler says:
<<Thanks for trying out Google Browser Sync and for all of your feedback. It was a tough call, but we decided to phase out support for Browser Sync. Since the team has moved on to other projects that are keeping them busy, we don't have time to update the extension to work with Firefox 3 or to continue to maintain it.>>