Basically Persona allows you to change your theme, (also known as skins), similar to when you change your instant messaging skin (AIM, Yahoo, etc). It is a way to spice up the look of your browser, and individualize the feel according to your preference. I think Personas came out several months ago with the release of Firefox 3.6, or at least the ability to update your Persona really easily without installing any additional plugins add-ons.
All you need to do is, roll over any theme in the Personas Gallery (http://www.getpersonas.com) to preview what your browser would look like with that them, and then click on “Wear this Persona” to instantly apply the skin. They say that there are currently over 35,000 designs to choose from, including music artists. To remove, go to Tools -> Add-Ons -> Themes -> Uninstall
If you want browser access to the Personas themes, there is a Firefox Add-On called Personas Plus?https://addons.mozilla.org/en-US/firefox/addon/10900? where you can browse through over 180,000 skins.
If you’re feeling creative, you can also create your own persona. Here’s directions on how to: http://www.getpersonas.com/en-US/demo_create
Now you can look even cooler, browsing the web. But of course, just like any skins, it has some disadvantages in term of user interface depending on design. Depending on the theme that you choose, you may have a hard time seeing the browser buttons and text on top, including the address bar. There seems to be some complaints against Firefox, but they didn’t create the user submitted themes. If you’re having a hard time seeing buttons, choose a theme with a contrasting background color and text. If you design Persona and wants to create some awesome user-friendly themes, I would suggest following some good design standards, like the web standards, contrasting colors of text to background. Your theme users may be more happy.