Google has just released a Firefox extension that "continuously synchronizes your browser settings". Basically it will store the settings you choose in your Google account — which are then loaded each time the browser is launched. I just installed it on my laptop and in PortableFirefox. It does require that you have a GoogleMail (G-mail) account but gives you the option to set one up. The settings are stored in your Gmail account. It works better than any other synching options I have seen.
"Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings – including bookmarks, history, persistent cookies, and saved passwords – across your computers. It also allows you to restore open tabs and windows across different machines and browser sessions."