If Mozilla creates their own distribution of Android and sells phones with it, then they're extremely relevant in the mobile space. Take a look at the principles of the Mozilla manifesto[1]. Nowhere does it say, 'we need to have a relevant web browser'. It's a bigger mission about an open internet, unrestrained communication, libre software and ultimately end-user sovereignty. When Mozilla.org was created, the best way to achieve those goals was by having an awesome libre browser. As you rightly point out, the world has gone mobile, and the way for Mozilla to continue their mission is by releasing a mobile OS built with their principals in mind. If this was 2007, building Firefox OS may have been a viable strategy. But it's 2015, and the quickest, and in my opinion the only, way to succeed is with a distribution of Android with Mozilla services.