user.js Syntax

Understanding user.js syntax

The user.js is a list of instructions you're giving to Firefox every time you restart the browser. Each instruction tells Firefox to set a preference to a specific value.

Basic Structure

Every line that sets a preference follows this format:

user_pref("preference.name.here", value);

• user_pref: A command that tells Firefox, "Hey, I want to set a user preference." You'll see this at the start of almost every active line.
• ("preference.name.here", value): This part is enclosed in parentheses.
  • "preference.name.here": The name of the Firefox setting you want to change. It's always a string of text, wrapped in double quotes. These names can look a bit like website addresses, with dots separating different parts. For example, browser.startup.homepage tells Firefox what page to load when it starts.
  • ,: A comma separates the preference name from its value.
  • value: What you want to set the preference to. The type of value depends on the preference (see next section).
• ;: Each user_pref line must end with a semicolon. This tells Firefox that the instruction is complete.

Types of Values

There are three main types of values you'll use:

1. Boolean (True/False):

   • Designate as either on or off.
   • You'll use true (to turn it on) or false (to turn it off).
   • Example: user_pref("browser.tabs.warnOnClose", false); (This tells Firefox not to warn you when you close multiple tabs.)
   • Important: Don't put quotes around true or false. They are keywords, not text strings in this case.

2. Integer (Numbers):

   • Require a whole number.
   • Example: user_pref("network.http.max-persistent-connections-per-server", 6); (This sets the maximum number of connections to a server to 6.)
   • Important: Just use the number. No quotes.

3. String (Text):

   • Require text, like a URL or a file path.
   • Strings must be enclosed in double quotes.
   • Example: user_pref("browser.startup.homepage", "https://www.duckduckgo.com"); (This sets your homepage to DuckDuckGo.)

Adding Comments

Sometimes you want to leave notes in your user.js file to explain why you made a change, or to temporarily disable a setting without deleting it. You can do this with comments. Firefox ignores comments.

• Single-line comment: Start the line with //. Everything after // on that line is ignored.

  • // This is a comment explaining the next line.
  • user_pref("some.setting", true); // This part is also a comment.

• Multi-line comment (less common in user.js but good to know): Start with /* and end with */. Everything between them is ignored.

  • /*
  • This is a comment
  • that spans multiple lines.
  • */

Common Problem Areas

• Typos: A misspelled preference name or an incorrect value type will mean the setting won't apply.
• Missing Semicolons: Every user_pref line needs to end with ;.
• Quotes: Use double quotes for preference names and string values. Don't use them for booleans (true/false) or integers.
• Activating Changes: Firefox only reads the user.js file when it starts up. So, if you make changes, you must restart Firefox to see them take effect.

Betterfox defaults

The Betterfox project is a curated collection of these user_pref settings. We've put a lot of thought into these defaults, but feel free to make it your own.