You could say that without many users, there won't enough real-world exposure, but at the same time, Firefox will also be targeted far less, so you wouldn't have to worry about security vulnerabilities as much.
And while yes, if at some point Chrome becomes Internet Explorer 2.0 and webpages target nothing else anymore, that could cause Firefox to be hardly usable, but that's then an entirely different nightmare and not anymore relevant to the current situation.
With no devs, you can’t implement those features or the security easily.
And especially for security you’ll want fulltime employees.
Also, you’ll have to pay more than Google pays their Chrome devs to ensure the people will keep working for you.
But maintaining a whole OS with a whole virtualization layer, a whole sandbox system, several supported scripting systems, its own scheduler, its own graphics stack, and support for compatibility with any bug a competing system has?
Modern browser are reaching complexities we’ve only seen in whole operating systems before. You don’t see a bunch of volunteers maintain everything from Linux to KDE at once, it’s hundredthousands of people working in a very different way.
To be able to compete against Google, be able to force them to implement features you pioneer, and force them to avoid implementing other features, to be able to keep up with them and reach better security than them, the current Firefox team is not enough.