Personas, its Initial release was not too late (December 14, 2007) but it was too late to be built in Firefox (from version 3.6, January 2010). In the earlier years people even used background image in IE6.

From version 29 Firefox uses Australis UI, with tabs on title bar and the simplified interface, Personas will no longer be useful. It will quickly be "a product of that age" I guess.