Personas3d is in beta.
Until the script is fixed, you MUST Preview before trying to Wear or Download. The Preview is currently very slow, in the future it should only process part of the image.
To load the generated persona with the Wear button, you need to add hwi.ath.cx to extensions.personas.authorizedHosts in your about:config options. Otherwise you must download the image and make a custom persona.
How to get Header/Footer URLs:
If you have Unix/Cygwin/bash and ImageMagick>=6.4.4, you can process your persona at home using the Personas3d shellscript.
When I tried Firefox 3.6 I found Personas3d was broken yet again. It seems I have to fix it every single upgrade, but this time I just could not be bothered. I downgraded to Firefox 3.5 instead :P If you can find the problem feel free to contact me.