The biggest feature debut in Firefox 17 (download for Windows | Mac | Linux) is the Social API, launching with Facebook Messenger, which allows participating social-networking sites to create persistent sidebar access to their site. Although most new browser technologies depend on entirely on how many developers integrate it, the Social API likely will be different.
To activate it, go to the Facebook Messenger for Firefox site and click "Turn On." It automatically adds Facebook control buttons to the right of your location bar, and makes Facebook's status update and messenger sidebar persistent even when you're not looking at Facebook.com. The Facebook button next to your Home button lets you control and hide the sidebar.
There are only a handful of heavily trafficked social-networking Web sites, and Mozilla is cautiously controlling the rollout. It's an atypical step for the company, but the integration is much tighter than normal. Facebook's integration, for example, obviates the need for social addicts to keep Facebook itself always open. The Social API is an interesting reaction to a more connected Web, but whether it actually will attract people has yet to be seen. Debuting with Facebook, however, means that competitors like Twitter and LinkedIn are likely taking a close look at the API.
Other changes in Firefox 17 include halting support for Mac OS X 10.5, and adding click-to-play blocklisting for add-ons. This will prevent out-of-date plug-ins like Adobe Flash and Apple QuickTime from loading without your permission. Another security change has been to implement the "sandbox" attribute for iFrames, which allows them to run more safely.
You can read the full list of changes for Firefox 17 here.