How to - Atavic/legacy-user.js Wiki

You can customize Mozilla-based products as Firefox or Palemoon using the UI Menu: select Edit -> Preferences

More options are available by typing about:config. All the changes you make are in bold fonts in the config screen. By double-clicking on a preference entry you reset it to the default value.

With Legacy-user.js file you apply modifications by putting the custom user.js file in the profile folder. Place the modified user.js file in one of the following folders (note that <profile folder> and <username> change in your setup):

Android: /data/data/org.mozilla.firefox/files/mozilla/<profile folder>

For Android also see:

Unix-like (Linux): ~/.mozilla/firefox/<profile folder>

The .mozilla folder is hidden by default.

Mac (may be in one of the following locations):

~/Library/Application Support/Firefox/Profiles/<profile folder>
~/Library/Mozilla/Firefox/Profiles/<profile folder> 

The tilde (~) refers to logged user's Home folder.

Windows: C:\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\yourprofile\

Windows(portable): [firefox directory]\Data\profile\

Legacy-user.js is then read when the browser starts. Every preference in the file will be written to the prefs.js file. Note that removing a setting from user.js will NOT remove it from prefs.js. You need to change the setting in the UI, the about:config editor or edit the prefs.js file.

Other options: