once they accomplished that goal, they didn't know what to do next.
Actually, they did know what to do next, but they do so rather aimlessly and pathetically. It seems there's nobody at the wheel.
Mozilla helped get PNG adopted as an alternative to GIF, but that's only for still images, while GIF also does animations. Firefox first failed to promote MNG, then ensured its death by removing it from their browser, and much later introducing their own MPNG standard, which then repeated the above cycle of indecision, neglect, and sabotage.
Much the same goes for video... They shouted loudly in support of open video standards, but were pretty slow to even just include them in their own browser(s).
Personally, I'd be happy if Mozilla just focused on optimizing the hell out of Firefox. It's much slower and memory-hungry than other browsers on Android. Even on my desktop, a little bit of JS can drag the browser to an unresponsive crawl, even utilitarian sites like Amazon get less usable with each heavier and slower browser release.