It's a browser plug-in that lets you quickly apply different skins to your browser's chrome. You can also set your skin to change automatically with incoming designs on a JSON feed. The aim is to make new skins easy to build, and easy to try out without downloads and relaunching. They don't change any of the buttons in the browser like real Firefox Themes do. It's a good idea, but unfortunately most of the "personas" at launch are ugly as sin.
The labs blog says this is just the simplest in what will be a long list of increased personalization options in the future. This one is fun and worth checking out, but ultimately it's more than a little underwhelming.
It would be nice if users were able to upload their own background image. The term "Personas" seems a bit overstated given what's actually being possible. As a proof of concept though, this one's worth a few minutes to check out.