Firefox Quantum‘s interface is still extremely customizable thanks to its userChrome.css file. You can edit this file to hide unwanted menu items, move the tab bar below the navigation toolbar, view multiple rows on your bookmarks toolbar, and do other things that normally wouldn’t be possible.

How This Works

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Firefox’s userChrome.css file is a cascading style sheet (CSS) file that Firefox uses. While style sheets are normally applied to web pages, this particular style sheet is applied to Firefox’s user interface. It allows you to change the appearance and layout of everything surrounding the webpage iteslf. You can’t actually add any features; you can only modify what’s already there to change, hide, or move it.

This has nothing to do with Google Chrome. “Chrome” refers to the user interface of the web browser, which is what Google Chrome was named after.

The userChrome.css file has existed in Firefox for a long time, but it’s taken on renewed importance with Firefox Quantum. Many tweaks that could previously be accomplished by browser add-ons can now only be accomplished by editing the userChrome.css file.

Where to Find Tweaks

While you could create your own tweaks if you understand CSS code and how Firefox’s interface was designed, you can also just find tweaks online. If you want to make a certain change, someone else has probably already figured out how to do it and written the code.

Here are some resources to get you started:

Sample Tweaks from userChrome.org: A short list of interesting tweaks that demonstrate the power of userChrome.css.

Classic CSS Tweaks: A repository of userChrome.css tweaks from the author of the Classic Theme Restorer extension, which no longer functions on Firefox Quantum.

userChrome Tweaks: A collection of interesting Firefox tweaks.

FirefoxCSS on Reddit: This subreddit is a community for discussing tweaks. You can search the subreddit to find other people’s tweaks, see what people are sharing, and even ask for input if you can’t find a tweak you really want.

Guide to Editing Your Context Menu: Instructions for removing items from Firefox’s context menu and changing their order in the list, taken from the FirefoxCSS subreddit.

Bear in mind that older versions of Firefox had a different interface. Older userChrome.css tweaks you find online may not function on Firefox 57 and later, also known as Firefox Quantum.

If you know what you’re doing with CSS, you can enable the browser toolbox to inspect the Firefox browser’s chrome. This will provide the information you need to customize various browser interface elements with your own CSS code.

How to Create Your userChrome.css File

The userChrome.css file does not exist by default, so once you have a tweak or two you want to try out, you first have to create the file in the appropriate location inside your Firefox profile folder.

To launch your Firefox profile folder, click menu > Help > Troubleshooting Information in Firefox.

Click the “Open Folder” button to the right of Profile Folder to open it. (On macOS or Linux, you’ll see a “Show in Finder” or “Open Directory” button instead. The following instructions show the process on Windows, but it’s basically the same on Mac and Linux—you’ll just be using a different file manager and text editor.)

If you see a folder named “chrome” in the profile folder that appears, double-click it. However, you probably won’t, as this folder isn’t created by modern versions of Firefox.

To create the folder, right-click in the right pane and select New > Folder. Name it “chrome”, press Enter, and then double-click it.

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You’ll need to tell Windows to show you file extensions, if you haven’t already. Windows hides file extensions by default to simplify things. This step isn’t necessary on macOS or Linux, which show this information by default.

On Windows 8 or 10, you can simply click the “View” tab on the ribbon and check the “File name extensions” box to make them visible. On Windows 7, click Organize > Folder and search options, click the “View” tab, and uncheck “Hide extensions for known file types”.

You will now create the userChrome.css file, which is really just a blank text file with the .css extension instead of the .txt extension.

To do so, right-click in the right pane here and select New > Text Document. Name it “userChrome.css” making sure to remove the .txt file extension.

Windows will warn you that you’re changing the file’s extension and this may be a problem for certain types of files. Click “Yes” to confirm your change.

On macOS or Linux, create an empty text file with the same name.

How to Edit the userChrome.css File

You can use any text editor to edit the userChrome.css file. The Notepad text editor included with Windows works just fine. If you want a more powerful text editor with more features, we like Notepad++.

To edit the file in Notepad, right-click it and select “Edit”.

Add whatever tweaks you want to the file by copying and pasting them in. If you add multiple tweaks, be sure to add them all on their own lines.

Once you’re done, save the file by clicking File > Save in Notepad.

Whenever you edit your userChrome.css file, you will have to close all open Firefox windows and relaunch Firefox for your changes to take effect.

If you find yourself coming back to the “chrome” folder frequently to edit your userChrome.css file, you may want to create a desktop shortcut to the folder or add it to the “Quick access” folders in File Explorer.

The userContent.css File

Firefox also has a userContent.css file you can edit, and you may stumble across some tweaks that say they’re for the userContent.css file.

To use this file, just create a file named “userContent.css” in the same folder as your Chrome folder. Tweaks you place in this file affect Firefox’s internal “content pages”, like the New Tab and Options pages.

Help, I Broke Something!

If you ever encounter a problem with a tweak, you can just remove it from your userChrome.css file and restart Firefox. If that doesn’t work, you can completely delete the userChrome.css file and restart the browser to erase all your changes and get a fresh Firefox interface.