I cannot separate web development and mobile development. Mobile devices will become more and more like Desktop devices : less native applications, more web applications. Even if Firefox OS has given up, the work of Google on Progressive Web Apps and the work of Mozilla, Google and Microsoft on Service Worker mean they will be ok to deploy more and more mobile web application.
Mobile device is a very specific platform as the internet connection is very capricious, and even with its quad-core processors, it is small and has less power than a classical laptop. Keeping that in mind may let you being a great mobile web developer.
PS: hope my english worth reading.