Browser integration is actually supposed to be one of the core pieces of Persona. The idea is that by building the Persona login process directly into the browser (as opposed to it requiring a popup/webpage) then phishing attacks may be somewhat mitigated.

That, and a better user experience. Have a look at this screenshot, doesn't it seem obviously more attractive and usable than a pop-up? http://www.extremetech.com/wp-content/uploads/2011/07/firefo...

Also, it's more private. With a native browser implementation, you don't communicate with persona.org every time you log in to a website, you only have to trust your browser to store your cached authentication credential.