The first thing they made was a web-browser, because they had all the pieces and all the people right there, they had a good idea how to do it, and because the biggest threat to openness, innovation and opportunity on the Web at the time was the dominance of Internet Explorer.
These days, Mozilla must be less focussed because there's no single dominant threat to focus on. There's things like video patent codecs, DRM, promoting HTTPS, the ever-increasing complexity of the browser stack, the enticing power of native mobile apps, the lack of an awesome funding model for freely-distributed content, the return of walled-garden content platforms...
They could focus on making Firefox awesome and abandon all the rest of those issues, then we'd have a nice browser but we might have a much less interesting and useful Internet to browse with it. I do not envy Mozilla's leadership in the slightest right now, they're making a bunch of very tough decisions and nobody will know if they did the right thing for maybe five or ten years.