The recent release of Firefox has a redesigned about:config page. If you miss the old one, you can get it back with this tip.

The configuration editor in Firefox (about:config) lets you access and change advanced settings and preferences that aren’t available from the Options menu, allowing Firefox to be customized to a great degree.
Here are a few of the things you can change in the about:config menu:

• Enable spell checking in all fields.
• Open search bar results in a new tab
• Open bookmarks in a new tab
• Disable Unnecessary Animations
• Disable Delay Time When Installing Add-ons
• Stop Firefox from opening a window when you click on a link in an email program

Some of these configurations change or disappear over time. The most accurate (if dated) site to discover about:config entries is here:
About:config entries - MozillaZine Knowledge Base

Changes are made by altering or creating parameters in the about:config menu. Before making any changes, backup your prefs.js file in your Firefox profile folder. To locate your profile folder, click or tap the menu (upper right, three bars), go to Help, and choose Troubleshooting Information. Select the Open Folder button next to Profile Folder and the folder will open in Windows File Explorer.
Copy the prefs.js entry to a different location or rename the prefs.js file to something like prefs.js.old.
To reset all entries to their original values, delete the prefs.js in your Profile folder, and then restore the folder, or if you've renamed it, undo the change. Individual entries can be restored to their default by right clicking the entry and choosing Reset.

How to show the old about:config menu:
To display the old about:config interface, in the address bar type: chrome://global/content/config.xul.
The list of entries displays immediately, there's no warning page. If you want to continue to use the old interface you can bookmark it (Ctrl + D) instead of typing in the address every time.

The new about:config menu is opened by typing about:config into the browser address bar and pressing enter. A large Proceed with Caution appears. Click the Accept the Risk and Continue box below it to get to the next window. Once there you’ll see a notification that “Changing these preferences can impact Firefox performance or security”.
At that point you can search from the search box at the top of the page or select Show All to see the entries listed alphabetically.

Changing about:config settings can cause issues with Firefox. It’s a good idea to change one entry at a time unless you’re familiar with the about:config menu.

There are more “about” menus such as about:addons, about:plugins, about:memory and more. You can find a list of them here:
List of All Mozilla Firefox about commands
These are "about" areas, which display information on various about sections, not the list of about:config entries. Some are quite useful, the about:memory can help monitor use.

The new version of the about:config menu has larger type size, but requires an extra click or tap to get to. The old version skips the warning and goes straight the entries. Use one or both to customize Firefox to your liking.

You can find more Tech Treats here.