That always depends on the market. While Chrome leads the US market like IE used to, Firefox is for example the most-used browser in Germany. Maybe it would have been a valuable approach for Mozilla to work with a specific market when introducing Persona, e.g. the big German webmail providers.
What if there was a way to "spin out" Persona from Mozilla? How many devs and how many resources would it take for a bare-bones Persona Foundation to get the job done?