Jake of the Web

http://www.google.com/tools/firefox/browsersync/
Made by Google, BrowserSync synchronizes your browser settings –
including bookmarks, history, cookies, and saved passwords – across
your computers. It also allows you to restore open tabs and windows
across different machines and browser sessions.
You do need a Google account in order to use this – because the data
is stored on Google's servers.
It's independent of OS – that is, you can synch browsers on a Mac and
a PC and on a machine running Linux, as long as you are using Firefox
1.5 or greater on all three.