This article is no longer maintained, so its content might be out of date.
I used to be able to use a Persona and a Theme at the same time, but now I can't.
That was a happy accident. The Personas extension was never meant to support the use of Personas and Themes at the same time. That just happened to work in a few cases. In order to ensure Firefox stability and functionality, Firefox 3.6 does not allow Themes and Personas to be used together.