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The Firefox GNOME theme has reportedly been updated to keep its GNOME-style custom CSS working with Firefox 157’s Nova interface. The report describes a compatibility maintenance update, not a new official Firefox theme or an update distributed through Mozilla Add-ons. The project’s captured README and releases page do not yet confirm a Nova-specific release, so check the current project release or commit before installing.
What changed with Nova in Firefox 157
Mozilla introduced Nova in Firefox 157, which it says shipped on September 29, 2026. The interface refresh changes the browser frame, gives the sidebar more prominence and makes vertical tabs a first-class layout. In particular, the toolbar and sidebar sit flush with the window edges; with vertical tabs, the tab-bar color also appears on the toolbar, and the sidebar shares the toolbar area’s theme background. Mozilla’s Nova theme guidance explains that these changed surfaces can affect how existing themes look.
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Mozilla says, “Your theme continues to load, and most color and image properties work as they did before.” That does not mean every theme will look identical: properties may appear in different places, and some may no longer have a visible effect. Themes that rely on background images are especially likely to look different. Mozilla advises testing the theme in Firefox 157 and checking the affected surfaces.
What the reported GNOME theme update covers
OMG! Ubuntu reported on October 7, 2026 that the Firefox GNOME theme team updated its CSS for Nova-era class changes and new widget styling. The stated aim is to preserve the existing GNOME look. This is a secondary-source report; it does not establish that every Firefox build, package format or profile works, and there has been no independent installation or visual test to confirm the result.
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The project is an unofficial collection of CSS installed in a Firefox profile, rather than a conventional theme obtained from Mozilla Add-ons. Its README describes it as “a bunch of CSS code to make Firefox look closer to GNOME’s native apps” and says it follows GNOME Adwaita styling. The project also warns that some styling can depend on behavior Firefox upstream does not support. The project README is the source for its purpose and setup guidance.
Why the project’s version information needs checking
The available project pages do not independently verify the Nova update. The captured README says the master branch supports Firefox stable 150, while the releases page lists v149.1 as its latest release. The pull request page shows later Firefox compatibility work, including a July item concerning Firefox 151+, but does not establish which release or commit contains the Nova-specific changes.
Because those project records conflict with the October 7 report and may be stale, do not assume that a release number or branch label proves Nova compatibility. Before updating, check the current releases and recent project changes for a relevant update, then confirm that the instructions match your Firefox version and profile.
What to check before installing or updating
- Confirm the current version or commit. Look for a project update that addresses Firefox 157 or Nova, rather than relying on the older README support statement or release listing.
- Check your Firefox package and profile. The README’s installer says it detects common profile locations, including Flatpak and Snap paths, and it also documents manual installation. Verify that the current installer supports your package and that you are editing the profile used by your Firefox installation.
- Enable profile custom CSS. The project instructions require the Firefox preference
toolkit.legacyUserProfileCustomizations.stylesheetsfor custom stylesheets to load. Follow the current README for the exact steps, as preferences and profile paths can change. - Restart Firefox after installation. The project recommends a restart for the stylesheet to take effect.
These are the project’s documented instructions, not independently tested installation steps. For the current procedure, use the README rather than relying on an old copy of its setup directions.
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How to check whether the update works for your layout
- Start Firefox with the updated CSS in the profile you intend to use.
- Inspect the toolbar and sidebar, including their edge alignment and shared background.
- If you use vertical tabs, check the tab-bar and toolbar colors together.
- Look for misplaced, missing or unexpectedly persistent styling, especially where the theme uses background images or custom widget styles.
- If the result is broken, disable or remove the custom CSS using the project’s current instructions, then check for a newer compatible commit before trying again.
These checks focus on the surfaces Mozilla says may change under Nova; they are not a guarantee that every CSS feature in the project has been verified.
Optional theme settings are not Nova requirements
The README also documents optional preferences for hiding the tab bar when only one tab is open, placing tabs in the headerbar and using an OLED-black variant. They are separate customization choices, not prerequisites for Nova compatibility. Some options interact, and the project warns that its headerbar dragging feature is buggy. Consult the README before enabling them.
Quick Recap
Is this the right theme for you?
- Choose it if you want Firefox to resemble GNOME’s Adwaita-style native apps and are comfortable maintaining profile-level CSS.
- Check it carefully if you rely on vertical tabs or the sidebar, since those are among the Nova surfaces whose appearance changed.
- Verify package and profile support if you use Flatpak, Snap or another non-default installation; installer coverage should be confirmed against the current project instructions.
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