firefox-gx rebuilds the Opera GX look in CSS for Firefox

The install is two files in a profile directory

The installation instructions are four steps and the whole thing is a file copy. You download the theme from the releases section, unzip it, and copy the `chrome` folder and the `user.js` file into your Firefox profile directory. Then you install and use a compatible theme, and restart the browser. The profile directory location is found through about:support or about:profiles, and the README points at a section with an example of using about:support to reach the profile folder. There is a separate GX-Theme-Styles extension, made by the theme's author, that lets you switch between themes and colours quickly, though it works for solid themes only or for users of the Main-Image extra. The Firefox Color extension is suggested for changing the accent colour.

A family of colourways rather than one theme

The theme list is organised in three tiers and the naming makes the intent obvious. The main themes are Fuchsia, Blue, Green, Poison, Swamp in autumn, Red-Blur and Purple-Sky. The secondary colours are Purple, Aquamarine, Ember Orange, Frozen Cyan, Level Up Green, Stamina Yellow and Wizard Grey. The white themes are a Fuchsia variant and Waves. Beyond the palette, there are dark and light variants with the same names, and the way to move between them is a separate mechanism: two long theme URLs are given for the dark and light themes, and the instruction is to use the Advanced colors tab and change the colour in icons attention, which sets the accent colour to the theme. There is also a video explanation linked for switching between light and dark mode for web content.

The Extras folder is where the layout changes live

The Extras directory holds optional features that are not part of the base theme, and each one changes the shape of the browser rather than the colours. OneLine puts the tabs bar next to the navbar. Tab Shapes gives the tabs a different appearance. Main-Image simulates an image behind the whole browser, which is the feature the theme-switching extension's extra mode refers to. Left Side-Bar moves the bookmark bar to the left of the window. TreeStyle Tabs adds compatibility with the extension that puts tabs vertically. Classic Sound Icon shows a rhythm image on tabs that have sound. The list also links out to two external partner projects, a Nighttab-based new tab extension in the GX style and a play/pause button extension. The pattern of separate extras is the same pattern the theme itself uses, and it means the base install stays conservative.

The stability warning is the first thing to read

There is a notice in the README that deserves to be quoted rather than paraphrased, because it changes how you should treat the repository. The latest code updated in the repository, as opposed to the downloaded zip, may work well only on beta or nightly versions of the browser. That is a significant caveat for a theme, and it has a practical reason. A userChrome.css theme works by overriding Firefox's own stylesheets, so it depends on the internal class names and DOM structure of the browser UI. When those change in a release, the selectors stop matching and the theme either degrades or breaks. Beta and nightly builds change earlier and more often than release builds, which is what lets a theme be developed against them, but it also means the code that works there is the code most likely to break on the next release.

The releases are numbered separately from the version

The release history shows a numbering scheme that is worth understanding before you look for a particular build. The GitHub tags are v.12.4, v.12.3 and v.12.2, while the release names inside them are FirefoxGX_Release_v.158, v.157 and v.156. So the tag sequence and the release sequence are different counters, and the second one is the one that increments on every build. The most recent is v.12.4 from 2026-10-14, with v.12.3 from 2026-09-28 and v.12.2 from 2026-09-14, so the project ships roughly every two weeks. The last push to the main branch was on 2026-10-15. The repository root holds the `chrome/` directory with the theme itself, the `user.js` file, the `Extras/` directory, a `ThemePreview/` directory, a `Tricks/` directory and readmes in French, Spanish and English, and the licence is MPL-2.0, which is a good fit for a set of CSS overrides.

What it is and what it is not

The project describes itself as a theme for Opera GX skin lovers, with complete compatibility with the stable version of the browser, and the primary language is CSS. That last phrase is in tension with the stability notice, and the honest reading is that the released zips target the stable browser while the repository branch moves ahead. The scope is worth being clear about too, because people arrive at this expecting something more. It is a userChrome.css and user.js overlay that changes how the Firefox chrome looks. It does not change how web pages render beyond the accent colour, and it is not a browser fork. What you get is the visual language of GX, with a set of optional layout rearrangements, in a browser that still behaves like Firefox.

Editorial conclusion

Adopt firefox-gx if you want the Opera GX layout in Firefox and you are comfortable editing your profile directory, since the whole install is a copy of two items into the profile. Do not adopt it expecting a stable Firefox to keep working, because the project warns that the latest code in the repository may work only on beta or nightly versions, so a Firefox release can break the theme. Verify first that the release you download matches the Firefox version you run, that you know where your profile directory is from about:support or about:profiles, and that the licence terms match what you plan to do with the theme.

Frequently asked questions

Official sources

1. Godiesc/firefox-gx on GitHub
2. Issues
3. License: MPL-2.0
4. README
5. Releases