I don't do those things, but that's not because it's a bad method, but because I don't care. If there was some bug in some system call, or if I needed to investigate why hardware is misbehaving, of course I'd read the Linux source. That's one of the great things about open source software; you don't have to treat it like a black box. You can just look at the internals and see how they work.
(And I have proof that I do actually do this; I had a misbehaving joystick, and the linux fork in my github contains the fix.)
Anyway, in conclusion, if you want to know what algorithm Firefox uses to process CSS selectors, just read the Firefox source code. If you don't give a damn, then don't do that instead. But if you care and you do nothing, that's just silly.
