The Facebook Messenger for Firefox we were promised when Firefox 17 made it into beta is now available. It's the first app/add-on to make use of Firefox's Social API and it's rather slick, if having Facebook "on" all the time is your thing.
Note that the app only works in Firefox 17 Beta or Firefox 18 and newer.
While you have to install the add-on, technically just click the "Turn On" button, Facebook Messenger for Firefox is built into the browser, it's not an add-on.
People may be surprised by this since it looks and feels like any other traditional add-on, down to the buttons in the toolbar.
In fact, the only way to turn it off is to go to the Options/Preferences menu and untick "Facebook Messenger for Firefox."
If you're familiar with the Facebook chat bar and the friends/messages/notifications buttons on the site, you won't have any trouble with the ones in Firefox, they're identical, they not only look the same, it's entirely possible that they share the same code.
While there are a few kinks to work out, like the lack of an option to hide the friend list with one click, Facebook Messenger for Firefox works and does it seamlessly. Facebook fans will be pleased.