Switching to the default theme when you install a persona is why you need to restart Firefox. If you would be using the default theme and would install a persona, a restart isn't needed. Also, by design personas are only enabled with the default theme. There was an experimental version of the Personas extension made back in the days of Firefox 3.6, but the results of allowing an add-on theme along with a persona didn't work out to good.

Thanks for your reply. It would seem that one can use Themes or Personas, but not both. They cancel each other out. That is the very thing I was wondering about when I clicked on FAQs, but Firefox's description seemed to imply that both can be enabled: "A Persona is a special type of theme that changes the look of your browser without changing the navigation buttons, toolbars, and menus." But in fact Persona does affect the graphics of a non-default Theme's navigation buttons, toolbars and menus. I think your description of how Themes and Personas are different needs to be amended. It is confusing and incorrect. Clearly state that Personas can only be enabled with the default theme. Thanks the opportunity to comment.

From [/kb/Personas#w_using-personas]

If you are not using the Default Firefox Theme or another persona, you will need to restart Firefox for it to take effect.

See also http://kb.mozillazine.org/Themes (Lightweight theme)