How to *rebuild" FF to enable/disable functionality in "about:config"?

I need to rebuild FF of the release edition to enable some settings in “about:config” along with their default values which I also want to change. Meaning, in the release edition they aren’t even available, and in the dev/nightly editions their default values that aren’t the ones that I suit me.

I’ve searched by the name of a setting, but it’s brought me to a non-cpp file and which I haven’t been able to figure out how to change it properly.

How to go about this?

What makes a setting enable/disabled? What specifies its default value? Is this covered in the documentation?