Ura Soul: the idea is that, eventually, the identity system will be completely decentralize and built into everybody's browsers. The current JS-based implementation is a shim until that happens. It'll find its way into Firefox properly soon.
First, thanks for developing this. I've installed and tested the Wordpress.org version and no problems there.
However, on my Elgg I installed, activated the Persona for Elgg 1.8 plugin. My Elgg is set up so that only the login page shows when users aren't logged in. I then logged out and tried to log in via Persona. The popup worked, I logged in via Persona but my Elgg login page kept redirecting to itself, effectively stuck in a loop. I had to flush the browser cache to break the loop, log in normally a deactivate Persona.
Hope you can work out what happened and why and fix it!
Please don't hesitate to contact me if you have any further questions. I'm only too pleased to help with your project.