There weren't any straightforward guides when I looked this up, and I even had to ask myself. But I just needed to put so and so together, get some feedback here, and voila! Hopefully this can work for you too, and could edit userchrome.css in your favorite editor, and see the changes in Firefox immediately. I tested that it works with @import url("folder/file.css");, and nested imports too (if folder/file.css contained @import url("Another folder/file.css");.

1. Install fx-autoconfig (I haven't tested it with other Firefox JS loaders), following the whole install section: https://github.com/MrOtherGuy/fx-autoconfig?tab=readme-ov-file#install
2. In the chrome/JS/ folder, create <any file name>.uc.mjs (I named mine refresh.uc.mjs) and paste the script below (it's slightly modified from this snippet): * The part containing @onlyonce is needed so fx-autoconfig loads it just once, rather than spawn a new instance of the script every time a new firefox window is opened.
3. Clear startup cache: https://github.com/MrOtherGuy/fx-autoconfig?tab=readme-ov-file#deleting-startup-cache
4. You may need to toggle the script. You can go to Menu Bar > Tools > userScripts.

Script

// ==UserScript==
// @onlyonce
// ==/UserScript==

// Script from here:  https://gist.github.com/jscher2000/ad268422c3187dbcbc0d15216a3a8060?permalink_comment_id=3259657#gistcomment-3259657
setInterval(() => {
    /*
       Code to paste and run in the Browser Console
       Requires devtools.chrome.enabled => true in about:config
       Tested in Firefox 68.0.1 on Windows
    */

    // Create references to APIs we'll use
    var ss = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Ci.nsIStyleSheetService);
    var io = Cc["@mozilla.org/network/io-service;1"].getService(Ci.nsIIOService);
    var ds = Cc["@mozilla.org/file/directory_service;1"].getService(Ci.nsIProperties);
      
    // Get the chrome directory in the current profile
    var chromepath = ds.get("UChrm", Ci.nsIFile);

    // Specific file: userChrome.css or userContent.css
    chromepath.append("userChrome.css");

    // Morph to a file URI
    var chromefile = io.newFileURI(chromepath);

    // Unregister the sheet
    if(ss.sheetRegistered(chromefile, ss.USER_SHEET)){
      ss.unregisterSheet(chromefile, ss.USER_SHEET);
    }

    // Reload the sheet
    ss.loadAndRegisterSheet(chromefile, ss.USER_SHEET);
}, 1000)

OC by @TheTwelveYearOld@lemmy.world

There weren't any straightforward guides when I looked this up, and I even had to ask myself. But I just needed to put so and so together, get some feedback here, and voila! Hopefully this can work for you too, and could edit userchrome.css in your favorite editor, and see the changes in Firefox immediately. I tested that it works with @import url("folder/file.css");, and nested imports too (if folder/file.css contained @import url("Another folder/file.css");.

1. Install fx-autoconfig (I haven't tested it with other Firefox JS loaders), following the whole install section: https://github.com/MrOtherGuy/fx-autoconfig?tab=readme-ov-file#install
2. In the chrome/JS/ folder, create <any file name>.uc.mjs (I named mine refresh.uc.mjs) and paste the script below (it's slightly modified from this snippet):
   • The part containing @onlyonce is needed so fx-autoconfig loads it just once, rather than spawn a new instance of the script every time a new firefox window is opened.
3. Clear startup cache: https://github.com/MrOtherGuy/fx-autoconfig?tab=readme-ov-file#deleting-startup-cache
4. You may need to toggle the script. You can go to Menu Bar > Tools > userScripts.

Script

// ==UserScript==
// @onlyonce
// ==/UserScript==

// Script from here:  https://gist.github.com/jscher2000/ad268422c3187dbcbc0d15216a3a8060?permalink_comment_id=3259657#gistcomment-3259657
setInterval(() => {
    /*
       Code to paste and run in the Browser Console
       Requires devtools.chrome.enabled => true in about:config
       Tested in Firefox 68.0.1 on Windows
    */

    // Create references to APIs we'll use
    var ss = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Ci.nsIStyleSheetService);
    var io = Cc["@mozilla.org/network/io-service;1"].getService(Ci.nsIIOService);
    var ds = Cc["@mozilla.org/file/directory_service;1"].getService(Ci.nsIProperties);
      
    // Get the chrome directory in the current profile
    var chromepath = ds.get("UChrm", Ci.nsIFile);

    // Specific file: userChrome.css or userContent.css
    chromepath.append("userChrome.css");

    // Morph to a file URI
    var chromefile = io.newFileURI(chromepath);

    // Unregister the sheet
    if(ss.sheetRegistered(chromefile, ss.USER_SHEET)){
      ss.unregisterSheet(chromefile, ss.USER_SHEET);
    }

    // Reload the sheet
    ss.loadAndRegisterSheet(chromefile, ss.USER_SHEET);
}, 1000)

In 2025, we rolled out one update after another, all aimed at making your browsing better — with more flow, speed, choice, and control over your information and experience. Your window to the internet, whether on desktop, mobile, or across all your devices, has gotten an upgrade this year.

Whenever you open Firefox, we want it to feel like it speaks your language and matches your style. This month, our mobile team is rolling out features inspired by community ideas, user requests and the small everyday moments that make browsing more delightful.

Almost a month after releasing version 145, Mozilla has launched Firefox 146, the latest update to its popular open-source web browser, now available for download.

The release brings notable platform updates. Linux users running Wayland now benefit from native fractional scaling support, improving visual clarity and rendering accuracy on high-DPI displays.

On macOS, Firefox enables a dedicated GPU process by default, isolating WebGPU, WebGL, and WebRender. This change prevents full-browser crashes from graphics faults, instead restarting the GPU process transparently to maintain session stability.

https://www.xuldev.org/misc/ucjs.php

https://www.xuldev.org/misc/ucjs.php

Hello everybody, i hope you are having a great day!. I have a problem with Youtube about the frames drops, in 1080p60fps i have drops but in 1080p normal don't. For system operative i'm using Fedora 43. And the Firefox version is the 145.0.1 64 bits. My specs are low: Intel i5 6200u with his integrated graphics (Intel HD 520) Already installed the extension h264ify, and i think i have all the correct drivers and codecs from the rpmfusion (ffmpeg, mesa freeworld) If u need more detail and information i can give it to you. Thanks for anyone that is interested in help me.

#firefox

I have seen folks talk about a pain point for using FF (or forks) being related to YouTube being super slow. The about:config settings the article mentions did seem to lead to YouTube loading faster on both my FF and Zen-Browser installs. So maybe this might help for others that specifically don't like to use FF as their main browser because of YouTube.

For those that just want the settings:

gfx.webrender.compositor.force-enabled (set to true and restart FF)

If on AMD GPU, this extra setting is supposed to help reduce CPU usage:

media.wmf.zero-copy-nv12-textures-force-enabled (set to true and restart FF)

"It’s safe to say that the people who volunteered to “shape” the initiative want it dead and buried. Of the 52 responses at the time of writing, all rejected the idea and asked Mozilla to stop shoving AI features into Firefox."

Now, we’re excited to invite you to help shape the work on our next innovation: an AI Window. It’s a new, intelligent and user-controlled space we’re building in Firefox that lets you chat with an AI assistant and get help while you browse, all on your terms. Completely opt-in, you have full control, and if you try it and find it’s not for you, you can choose to switch it off.

https://www.firefox.com/en-US/ai/