Google Browser Sync was a reasonably popular Firefox 2.0 add-on from Google Labs. Like Mozilla's
Weave (which is still having some early teething pains), Browser Sync was designed for people using Firefox on more than one machine, keeping bookmarks, history, and saved passwords the same on those machines. But you'll notice the past tense: for whatever reason, Google decided not to update Browser Sync for Firefox 3.0, and
discontinued the project last month.