firefox-user.js

Table of Contents

🟪 WHERE DOES IT GO
🟪 SYNTAX
🟪 USAGE

🟥 Summary: How a user.js works. To know how to apply arkenfox, read the rest of the wiki

Prefs are settings that control Firefox's behavior. Some can be set from ☰ Settings and all can be found in about:config, except for what are called hidden preferences which will only show when they are modified (i.e. set to any value by the user or browser - they have a trash can symbol for resetting)

🟪 WHERE DOES IT GO

See the Apply section

🟪 SYNTAX

A user.js file is a javascript file and is text based, and resides in your profile folder. It is used to set preferences for that profile when Firefox starts. You can update the user.js while Firefox is open, it is only ever read when Firefox starts.

Prefs must follow Mozilla's syntax which is user_pref("prefname", value);

the pref name must be wrapped in quotation marks
string values must be wrapped in quotation marks
prefs are case sensitive
a semi-colon is required at the end

user_pref("pref.name.string", "i am a string"); // strings require quote marks
user_pref("pref.name.boolean", true);
user_pref("pref.name.integer", 0);

user_pref("pref.name", "value") // this will NOT be applied, it is missing a closing ;
user_pref("pref.name.integer", "0"); // this will be applied but NOT do anything (type mismatch)

user_pref("browser.startup.page", 0); // this will be applied and actually work
user_pref("browser.Startup.page", 0); // this will be applied but NOT do anything (incorrect case)

/* this is a
   multi-line comment
   user_pref("preference.name", "value")
   and nothing in it is applied ***/

// two forward slashes indicate a comment
   // and do not need to be closed at the end
      // and only apply to the one line from when they are added
user_pref("pref.name.example", "value"); // comment starts at //

user_pref("pref.name.example", "not commented out"); // this is an ACTIVE pref and will be applied
// user_pref("pref.name.example", "commented out"); // this is an INACTIVE pref and will NOT be applied

🟪 USAGE

🔷 Firefox starts
- It reads the ACTIVE prefs in user.js, in order, and writes them to prefs.js
  - It ignores INACTIVE prefs, i.e it does not reset them
  - If a pref already exists in prefs.js, it overwrites it
  - If a pref is listed twice, the last one will be applied last
  - It may abort parsing the file if it encounters a syntax error, but will still apply any content up to that error
    - Not all syntax errors cause parsing to abort, check the browser console (Ctrl-Shift-J) for errors
  - It will not abort if you apply a type mismatch, it will actually write that type mismatch into prefs.js (and Firefox will ignore it)
- The user.js is now ignored until the next time Firefox starts
🔷 Firefox opens
- The contents of prefs.js are used and shown as modified in about:config
  - If a pref in prefs.js has a type mismatch, then it is ignored and the previous value is retained
🔷 about:config
- All values stored in prefs.js, even if they are the default value, will be denoted as modified
- If you set a value to a modified value, it is stored in prefs.js
- If you reset a value to default, it is removed from prefs.js
- If you reset a hidden pref to default, the value will be blank, and assuming it is not applied again from a user.js or by the browser, it will then vanish on the next about:config reload

🟩 Next: Backup