And what do you do after three years? Then the cash will be used up.
Mozilla isn’t just developing the Firefox browser. Technology is inherently political - and educating people and influencing actors politically on the free and open web is very important. Firefox is much less likely to mis-align away from their browser users than chrome simply because they don’t have the misaligned incentives like the chrome Browser which is equally made by the largest internet advertising firm of the world.
They even has created FirefoxOS for phone at some point in the past 10 years. But I don’t remember what happened with that.