How to display only modified preferences on about:config in Firefox

[harlan4096]

The internal about:config interface of the Firefox web browser is a handy tool to make advanced configuration changes to the browser including many that users would not be able to change otherwise and many that other browsers don't provide at all.

Mozilla launched a redesign of the about:config page in recent versions of the Firefox web browser; the new design uses open standards such as HTML and JavaScript and was introduced in early 2019 in development versions of the browser.

The initial redesigned version received some criticism as Mozilla launched it without full replication of functionality of the old version. Main points of criticism included that deep linking was no longer supported, that the data could not be sorted anymore, that all preferences could not be listed on the page, and that double-clicks to edit values or change states were not working as well.

Mozilla addressed some of these issues in recent updates. It is now possible to display all preferences and to use double-clicks. Sorting and deep linking on the other hand are not supported in Firefox Stable at the time of writing and Mozilla revealed earlier that it won't introduce these features.

Firefox users who want to take a look at all modified preferences have a new option now to display all changed preferences on about:config. The listing of core modified preferences on aboutupport displays only some of the preferences but not all that are modified.

Here is how you display all modified (non-default) Firefox preferences on about:config:

1. Load about:config in the Firefox address bar.
2. Confirm that you will be careful if the warning is displayed.
3. Activate the "show all" link to display all preferences.
4. Use the keyboard shortcut Ctrl-Shift-K (on Mac Command-Option-K) to display the Console of the Developer Tools.
5. Type allow pasting to enable the pasting of commands.
6. Paste the following in the console and hit the Enter-key afterwards.

var elements = document.getElementsByTagName('tr');
[...elements].filter(
el => !el.classList.contains('has-user-value')
).forEach(
el => el.style.display = (el.style.display === 'none') ? 'table-row' : 'none'
);

The instructions parse the data and display only preferences that have been modified. You can go through the listing easily this way to check all modified preferences in the Firefox browser. The change is temporary in nature, a reload loads the standard listing again.

Closing Words

It may sometimes be useful to check modified preferences, e.g. when something is not working right in Firefox and you are uncertain whether it is caused by one of the preferences. or when you want to make sure that certain preferences are still set to the values that you set them to.

Now You: If you use Firefox, do you use about:config?
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