In preparation for Firefox 3.6, due later this year and which will feature lightweight themes support or personas, Mozilla Add-ons has just added a section dedicated to personas, so users won’t have to go a separate site for all their customization needs.
Like GetPersonas.com, you just hover the personas thumbnails to live preview it on Firefox. If you like it, just press Wear it to keep it. To choose another theme, open the Add-ons Manager and select a different one in the Themes tab.
For the easiest way of creating and sharing personas, GetPersonas.com is still the place to go.