There is a lot of useful information about Firefox hidden away behind the about: URL protocol. The most useful is the URL about:config, which displays preferences and settings that can be inspected and changed.

Available about: URLs depend on your specific Firefox version. Here is a complete list of URLs in the about: pseudo protocol:

about: page Description
about:about Provides an overview of all about: pages available for your current Firefox version
about:addons Add-ons Manager
about:buildconfig Displays the configuration and platform used to build Firefox
about:cache Displays information about the memory, disk, and appcache
about:checkerboard Switches to the checkerboarding measurement page, which allows to detect checkerboarding issues
about:config Provides a way to inspect and change Firefox preferences and settings
about:compat Lists overriding site compatability fixes, linked to specific bug issues.
about:crashes Lists all crashes, which happened during the runtime of Firefox (in case the user enabled the crash reporter)
about:credits Lists all contributors to the Firefox project
about:debugging Switches to the Developer Tools debugging page, which allows you to debug add-ons, tabs and Service Workers
about:devtools Summarizes the developer tools and provides links to documentation for each tool
about:downloads Displays all downloads done within Firefox
about:home Start page of Firefox when opening a new window
about:license Displays licensing information
about:logo Firefox logo
about:memory Provides a way to display memory usage, save it as report and run the GC and CC
about:mozilla Special page showing a message from "The Book of Mozilla"
about:networking Displays networking information
about:newtab Start page when opening a new tab
about:performance Displays memory and performance information about Firefox subprocesses/add-ons/tabs
about:plugins Displays information about installed plugins
about:policies Lists out the Firefox for Enterprise policies
about:preferences Firefox settings (also available through Firefox menu > Options)
about:privatebrowsing Start page when opening a private window
about:profiles Display and manage Firefox profiles
about:protections Privacy Protections report consisting of Enhanced Tracking Protection, Firefox Monitor and Firefox Lockwise data
about:restartrequired A page users are sent to when Firefox requires a restart due to an update
about:reader Indicates a web page has Firefox Reader View turned on. See Firefox Reader View for clutter-free web pages
about:rights Displays rights information
about:robots Special page showing notes about robots
about:serviceworkers Displays currently running Service Workers
about:studies Lists the Shield Studies that are installed
about:sessionrestore Session restoration (displayed after a Firefox crash)
about:support Troubleshooting information (also available through Firefox menu > ? (question mark) > Troubleshooting Information)
about:sync-log Displays a synchronization protocol related to the Sync feature
about:telemetry Displays telemetry data collected and sent to Mozilla while Firefox is running (in case the user enabled telemetry)
about:url-classifier Displays the status of the URL Classifier services that Firefox uses (for example for Safe Browsing)
about:webrtc Information about WebRTC usage
about:welcome Page first displayed when Firefox is installed
about:welcomeback Information page displayed after Firefox is reset

These URLs are defined in docshell/base/nsAboutRedirector.cpp within the kRedirMap array. The array maps most of the URLs, like config to URLs in the chrome: pseudo protocol, such as chrome://global/content/config.xul. The about location information is duplicated in docshell/build/nsDocShellModule.cpp.

Creating your userChrome.css file is the first step on your quest to make over Firefox's interface. It takes a couple of minutes, but it's important to take your time and get it right so Firefox loads and applies your style recipes on the first try.

At startup, Firefox will check for the userChrome.css file in a particular location: a folder named chrome inside your currently active profile folder. The profile folder is where all of your current settings and add-ons are stored. (If you want to learn more about that folder, see "Profiles — Where Firefox stores your bookmarks, passwords and other user data" on Mozilla Support.)

Here's how you set that up (full details below):

  1. Open your currently active profile folder
  2. Create a new folder named chrome
  3. Create a desktop shortcut (alias) to the chrome folder for easier future access
  4. Make sure your OS is set to show you file extensions like .txt and .css
  5. Create a new text file inside the chrome folder named userChrome.css
  6. Change a preference in Firefox so it looks for your files at startup
 

(1) Open your currently active profile folder

The quickest way to open the profile folder in Firefox right now is to launch it from the Troubleshooting Information page. To access that page, you can use any of these methods:

  • Click the menu button (what I call the "3-bar" menu button), then Help ("?" button), then Troubleshooting Information
  • On the top menu bar, click Help, then Troubleshooting Information. To activate a hidden menu bar on Windows, tap the Alt key or the F10 function key.
  • Type or paste about:support in the address bar and press Enter/Return to load it.

In the first table on the page, find the Profile Folder row and click the button to the right, which will say "Open Folder", "Show in Finder" or "Open Directory" on Windows, Mac or Linux, respectively. This should launch a new window with your system's file browser (e.g., Windows Explorer). You should see numerous files and folders. On Mac, if Finder shows you a semi-randomly-named folder, double-click that folder to view its contents.

(2) Create a new folder named chrome

Normally, your profile does not contain a chrome folder unless you've been running Firefox for a very, very long time. To create a chrome folder, you can use the standard method provided by your file browser. For example, on Windows:

  • Right-click a blank area of the list, click New, then Folder. Type chrome (all lower case) as the name of the new folder, then press Enter or click away to complete the edit.
  • Click the New Folder button. Type chrome (all lower case) as the name of the new folder, then press Enter or click away to complete the edit.

On Mac:

  • Right-click (or Ctrl+click) a blank area of the list, click New Folder. Type chrome (all lower case) as the name of the new folder, then press Return or click away to complete the edit.
  • On the menu bar, click File, then New Folder. Type chrome (all lower case) as the name of the new folder, then press Return or click away to complete the edit.

(3) Create a desktop shortcut (alias) to the chrome folder for easier future access

It is very likely that you will need to add or modify files to the chrome folder in the future. I recommend adding a shortcut to the folder on your desktop for easier access. To create a shortcut, you can use the standard method provided by your file browser. On Windows, right-click the chrome folder, click Send To, then click Desktop (as shortcut). On Mac, Ctrl+click / two-finger-click the chrome folder, then click Make Alias. Drag the new chrome alias item to the Desktop.

(4) Make sure your OS is set to show you file extensions like .txt and .css

Out of the box, Windows and MacOS hide certain common file extensions to reduce the appearance of clutter. However, this makes it difficult to change a file extension when you want to. Since both .txt and .css are common file extensions, we need to (at least temporarily) turn off this feature.

In newer visions of Windows, where Windows Explorer has a "ribbon" at the top with a View tab, you can click View, then find the checkbox for "File name extensions" and check the box. In older versions of Windows, one of these methods should work:

  • Activate the top menu bar by tapping the Alt key or F10 key, click Tools, then Folder Options, then the View tab. Uncheck the checkbox for "Hide extensions for known file types."
  • If Windows Explorer has an Organize button on the toolbar, click Organize, then Folder and Search options, then the View tab. Uncheck the checkbox for "Hide extensions for known file types."

On Mac, click Finder > Preferences, then on the Advanced panel, check the box for "Show all filename extensions".

(5) Create a new text file inside the chrome folder named userChrome.css (or download a blank one)

Double-click chrome to open the chrome folder. The method you use to create the userChrome.css file may vary depending on the features provided by your file browser. Here's how you can do it in Windows: Right-click a blank area of the list, click New, then Text Document. Type userChrome.css — with only the C capitalized — in place of New Text Document.txt, then press Enter or click away to finish the edit.

For either Mac or Windows, you can use the following link to save the file into your chrome folder (or if it goes into Downloads, move it to the chrome folder): userChrome.css (blank file).

Congratulations, you're done creating the file. It's currently empty and awaiting a purpose in life. Firefox will learn about it the next time you exit out of Firefox and start it up again. (For Firefox 69, see step 6.)

Alternately, if you have some CSS code ready to paste into a userChrome.css file, you can do that using the following page and download your userChrome.css file from there: Generate Downloadable userChrome.css File.

(6) Set Firefox to look for userChrome.css at startup (updated 5/24/2019)

To make startup faster for most users, Firefox 69 will no longer look for this file automatically. You need to tell it to look. Here's how:

  1. In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
  2. In the search box above the list, type or paste userprof and pause while the list is filtered. If you do not see anything on the list, please ignore the rest of these instructions. You can close this tab now.
  3. Double-click the toolkit.legacyUserProfileCustomizations.stylesheets preference to switch the value from false to true.

That change should take effect on your next startup.

Adding Style Recipes to userChrome.css

When you're ready to add rules to the file, you may want to use a good text editor. There are suggestions for open source editors as well as information about using Firefox's Browser Toolbox, on these pages:

Source: https://www.userchrome.org/how-create-userchrome-css.html