Mozilla's revenue increased to over $500M in 2016. Good for them! Some people have asked why all that money, and Mozilla's 1,200-ish staff, are needed just to build Firefox.
In practice building and maintaining a truly independent browser is incredibly expensive. (By truly independent, I mean a browser with its own implementation of HTML, CSS, JS, etc; this kind of independence is critically important to the open Web, and the world, and hence Mozilla's mission.)
Why is it so expensive? First you have to do the obvious stuff: implement the client browser engine and interface. Web standards are vast and complicated and ever-evolving, so this is a vast and complicated task. For this kind of security-critical, long-lived software in an intensely competitive mass market, you have do a very high-quality job, paying attention to all the various "ilities" — security, compatibility, performance, portability, accessibility, internationalization, maintainability, and so on. But also these days a large server-side investment is needed: downloads, updates, addons, crash collection, telemetry, push notifications, sync, and so on. Then you have to support all your developers with CI, massive test infrastructure, analysis and viewing of crash and telemetry data, etc. You also support your developers by writing custom tools and improving open-source tools; for example the rr project was born at Mozilla to improve life for Mozilla's C++ developers.
If you care about the open Web standards you're implementing, then your people need to spend time doing standards work. Since you have an independent engine you need to constantly evangelise Web developers and manage other external relationships.
If you want to stay competitive for the long term you need something like Mozilla Research, building stuff like Rust and Servo to explore technology that may give you a competitive edge in years to come.
Your product needs to be in the hands of users to be relevant, so you need marketing, distribution, events and other kinds of user outreach.
Then of course you have all the people needed to let the above people do their work effectively — HR, PR, lawyers, accountants, logistics, office managers, personnel managers, executives, and so on.
I worked at Mozilla for a long time. Over the last five years headcount was at about the quoted level, even during the FirefoxOS years. We were constantly butting up against headcount limits, having more work than we had people to do it. 1,200 people is barely enough. In the future it might not be enough.
Of course if you're willing to give up on engine independence you can save a lot of those expenses by adopting another vendor's engine, probably Chromium, and drafting off Google's investment. It's no surprise that for-profit companies would take that path, and that only Apple and Microsoft, companies with deep pockets and deep fear of Google, have declined to do so (though even Microsoft has caved on Android). Of course, using the same engine as Chrome limits your options for competing against Chrome, but it's tough to argue that those extra options are worth more than the cost. This is one reason among many why Mozilla is so important: it's Mozilla's mission that justifies the costs of maintaining an independent browser engine.