One thing I tell a lot of Entrepreneurs I help, which helps with people seeking jobs especially in today's market, is to just focus on doing what you love and growing at it without worrying about if there's a "huge" demand for it or not. Just do whatever YOU want to do and maybe find some open source projects to participate in that will give you "real world" stuff to point at where you honestly shine with your passion.
The reason I say this is because a lot of people think they make theirself a better candidate by being more versatile and acquiring the experiences they need in order to create the "reality" needed about theirself to logically qualify for the most positions to increase their chances of being able to find a job.
The problem this creates is the minute we need somebody who's a wizard at some particular thing you wind up scaring everyone away when mentioning it and because they don't want to come off as "inadequate" we more or less have to read their "no" on our own through their inaction and or excuses. Then the people we encounter who actually have the right type of flame in them don't have the level of technical ingenuity required required for the role because they've chosen to focus on learning how to make todo apps in all the most popular platforms for the sake of logically qualifying for the most jobs. So the people who wind up getting that position are the people who decided to dive into that particular thing just out of their own passion and curiosity becoming more and more clever with what they create always raising the bar on what those things can do.
An example of what I mean, is how I've come across a lot of front end developers who are wizards in Adobe Illustrator and can draw extremely well yet rarely ever use or play with SVG because their jobs revolve around building e-commerce platforms and banking apps which don't require much vector art aside of maybe a responsive svg logo and icons. Meanwhile their "dream job" in programming is one where they're able to express more their art skills some type of way but they never focus on advancing in that area because they stay caught up in learning whatevers in the highest demand so they can feel more secure in the workforce.
So if you fall into that same pattern it's not going to do anything but keep you too occupied/trapped to dive into whatever you really want to do which in turn never yields any examples that says the things you want to be able to say about yourself in that arena. Besides that, just because a job isn't in "high demand" doesn't mean they don't exist and pay lots of money. In fact that may be the component that makes companies bid over your attention because of how few wizards there are on the market for that skill. Then on another note, you may well be the one to pioneer a completely new role that doesn't exist yet for anyone to ask for because they're not aware of the value it can provide to in turn ask for it.
In regards to you working at Mozilla... this could be the difference in you doing basic/typical updates and maintenance to their documentation pages like virtually any other developer who knows HTML and a functional extent of JS can manage doing, or being some fashion of "lead developer" on some new cool software/app/plugin that nobody but you can step up to.
So just stay true to yourself, explore what you love, grow at the things you value and don't worry about all the things you "don't qualify for", let the people who love those things worry about qualifying for them.
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