One of the oldest firefox extension I am using from long time is Google’s browser sync. If you access internet from multiple location like home, college, office, etc, you can install this firefox extension at all places.
Once installed it keeps synchronizing your firefox bookmarks, history, saved passwords, cookies & session (i.e. tabs & windows).
Do not worry much about security. The extension itself is protected by your Google account login and password plus a 4-digit PIN. Also sensitive data like cookies and password are by default encrypted.
The extension is free as like most Google services. If you have large number of bookmarks you may notice slight delay at first synchronization, but after that it works like a charm! 🙂
Additionally, if you have a dual-boot machine with any OS combination you can use this extension to sync firefox in every OS. Although on dual-boot machine there is slightly geekish way to do this which also sync installed add-ons, greasemonkey scripts and anything else which this extension skips.