Yes, this is the extension I have been waiting for. Using Firefox on both Windows and Linux daily means I have two sets of bookmarks. Now that is a pain. Of course, I could try and synchronise them manually.
But Google have gone and made a really nice extension called Google Browser Sync. It does exactly what it says on the tin; that is it synchronises your settings between different installations of Firefox.
Admittedly, at the moment it does take a bit longer to load up Firefox, because it has to check for changes to your profile, but Google say it will get better.
Corrected first link 15/06/2006.