What "Show Only Modified Preferences" Does in Firefox

This article explains the function of the "Show only modified preferences" checkbox located within the about:config advanced configuration editor of the Mozilla Firefox web browser. You will learn how this feature filters customized settings from default browser configurations, why certain preferences appear on this list, and how to use this tool to troubleshoot browser issues and reset changes.

The Core Function

The "Show only modified preferences" checkbox acts as a filter for Firefox's hidden settings. When you check this box, the about:config page immediately hides all default browser settings and displays only the preferences that have been altered from their original, out-of-the-box values.

Instead of scrolling through thousands of standard configurations, you are presented with a condensed list of active changes. Every preference on this filtered list will appear in bold text, indicating its status as "modified."

How to Access the Checkbox

  1. Open Mozilla Firefox.
  2. Type about:config into the address bar and press Enter.
  3. Click the Accept the Risk and Continue button on the warning screen.
  4. Locate the Show only modified preferences checkbox directly below the search bar and click it.

Why Preferences Appear as Modified

When you enable this filter, you will likely see many modified preferences that you do not remember changing yourself. A preference is flagged as modified for one of three reasons:

  • Manual User Changes: You directly edited the value inside the about:config menu, or you changed a setting in the standard Firefox Settings menu (which updates the corresponding about:config value in the background).
  • Browser Extensions: Add-ons and extensions often modify hidden preferences to function correctly.
  • System Operations: Firefox automatically updates certain preferences to track browser state, such as the last time an update occurred, your printer settings, or hardware acceleration configurations specific to your computer.

Practical Uses for This Feature

The "Show only modified preferences" checkbox is primarily used for browser maintenance and troubleshooting:

  • Reverting Accidental Changes: If you altered a setting in about:config that caused Firefox to behave poorly, checking this box helps you find the exact setting you changed without needing to remember its specific name.
  • Resetting Values: Once the list is filtered, you can easily restore any modified preference back to its original state. Simply click the Reset (arrow) button or Delete (trash can) button on the far right side of the preference row.
  • Auditing Extensions: If an uninstalled extension left behind unwanted changes to your browser's behavior, filtering by modified preferences allows you to spot and remove those leftover configurations.