no special details
(Note: Your "more system details" shows you have Firefox 11, not Firefox 5)
Go to http://www.getpersonas.com and try a persona. If it doesn't work, maybe you already have a theme installed that is preventing you from using a persona? Go to Firefox Add-ons Appearance tab and try enabling the default theme and try again. For more information, see Use themes to change the look of Firefox.
Valastiak, any luck with this?