Edit within the browser via about:config or add them in the user.js file, easier to make it permanent and portable

as the other comment said, you can put in user.js but more specifically, that goes in the root of wherever your profile is. for me, that is:

    /home/<user>/.mozilla/firefox/<name>/user.js

this file doesn't exist by default, there's caveats with how it is overwritten, etc. i don't know much