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Having programs steal or sit on RAM without using it is never a good thing. That's why it's called a memory leak, because it's as if the free memory is leaking away. And it gets deprived from other apps that might need it more than Firefox or chromium does.

Your idea only works if programs actually take only as much ram as they need and give it back when done, but they don't do that, they usually sit on it until it's pried from their cold dead fingers. That's what memory leaks are, and modern browsers these days are extremely prone to them.