The centrepiece of the just out Firefox 17 is the new Social API. With it comes... Facebook Messenger support baked right into your favourite open source browser.
Aside from making Firefox more social, version 17 comes with click-2-play blocklisting which prevents outdated/vulnerable plugins from running without user permission.
Ubuntu users should be getting their version upgrades tonight. Just fire up Software Updater if it hasn't already alerted you.
The release notes has the highlights while the full bug fix list has all the details.