Stop Firefox from Selecting Trailing Spaces on Double-Click

This article explains the mechanics behind the layout.word_select.eat_space_to_next_word preference in the Mozilla Firefox web browser. Specifically, it details how changing this hidden configuration setting to false alters the browser's text-selection algorithm to prevent it from highlighting the blank space following a word when a user double-clicks it, ultimately improving the accuracy of copying and pasting text.

The Default Word Selection Behavior

By default—particularly on Windows operating systems—double-clicking a word in Firefox highlights both the word itself and the single blank space immediately following it. This behavior is an intentional design choice meant to speed up text editing. If you select a word to delete it or cut and paste it elsewhere, grabbing the trailing space ensures you do not leave behind an awkward double space or accidentally merge two words together.

However, this becomes an annoyance when you simply want to copy a word to paste it into a password field, a search bar, or a code editor. The extra space comes along with the word, often invalidating passwords or requiring you to manually hit the backspace key after pasting.

How the Preference Works Under the Hood

Firefox uses the Gecko layout engine to render web pages and handle user interactions like text selection. The layout.word_select.eat_space_to_next_word preference is a boolean (true/false) flag that directly controls Gecko’s word-boundary detection algorithm.

We can break down the preference name to understand its exact function:

  • layout: Refers to the browser's layout and rendering engine.
  • word_select: Refers to the specific action of selecting a whole word (triggered by a double-click).
  • eat_space_to_next_word: Refers to the action of "consuming" or including the whitespace that separates the selected word from the next word in the sentence.

When you double-click a word, Firefox scans the text to find the beginning and end of that word based on character types (letters and numbers) and delimiters (spaces and punctuation).

When this preference is set to true, the browser identifies the end of the word and then executes an additional step: it extends the selection boundary to the right to "eat" the whitespace up to the start of the next word.

When you change layout.word_select.eat_space_to_next_word to false, you instruct the Gecko engine to skip that final extension step. The selection algorithm identifies the first and last characters of the word, highlights them, and immediately stops. The whitespace is treated strictly as a boundary marker rather than part of the selection target.

How to Change the Setting

To stop Firefox from selecting the trailing space, you must modify this preference in the browser's advanced configuration page:

  1. Open Firefox and type about:config into the address bar, then press Enter.
  2. Click the button to accept the risk and continue.
  3. Copy and paste layout.word_select.eat_space_to_next_word into the search bar at the top of the page.
  4. Double-click the preference in the results list (or click the toggle icon on the far right) to change its value from true to false.

The change takes effect immediately without needing to restart the browser. The next time you double-click a word, the highlight will stop precisely at the last letter of that word.