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>/.

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