That's a good question; I'm in the 'external' camp. But other browsers such as Edge and Chrome have built-in support, so there's a percentage of Mozilla management and user base that would regard a browser as feature incomplete if it didn't have it.
At least, once the scaffolding is done, they can hand off development to the Chrome team and thence focus on the core browser issues of implementing HTML 5.x
Any modern system can read PDFs. Why does it need to be in the browser, instead of handed off to the app of the user's choosing?