Firefox Transparent Theme - Windows 11 Mica for Firefox

A transparent Firefox theme that puts the real Windows 11 Mica backdrop behind the browser toolbar. No fake blur image, no lightweight theme add-on - just userChrome.css and a handful of about:config prefs, so the desktop wallpaper tints the chrome exactly the way Explorer, Settings and other native Windows 11 apps do.

Built on top of FoxOne by Firnschnee - its one-line layout is what makes a transparent toolbar look good in the first place.

Русская версия / Russian README

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Features

• Mica transparency on the toolbar, window frame and tab strip - the genuine DWM backdrop, not a screenshot trick
• One-line browser UI - tabs, address bar and buttons share a single row (inherited from FoxOne)
• Transparent about:blank and empty tabs, so new tabs show the Mica backdrop too
• Hover-reveal bookmarks bar - collapsed to a 2px sliver, expands when the pointer enters the top row, animated so the page does not jump
• Readable popups - menus, panels and the URL-bar dropdown stay solid, because text over Mica is unreadable
• Dark chrome, light websites - the browser UI is dark regardless of the Windows theme, while pages keep their normal light scheme
• Clean URL bar - bookmark star hidden, clutter icons revealed on hover
• Ships with a ready-to-run portable build if you would rather not configure anything

Requirements

• Windows 11 (build 22621 or newer). Mica does not exist on Windows 10, macOS or Linux - see Other platforms
• Firefox 153+ with the Nova UI (tested on 153.0.3)
• Transparency effects enabled in Windows: Settings -> Personalization -> Colors -> Transparency effects

Installation

Option A - ready-made portable build

Download the archive from Releases, unpack it anywhere and run FirefoxPortable.exe. Theme, prefs and toolbar layout are already applied - nothing to configure.

Option B - apply to your own Firefox

1. Close Firefox completely (verify in Task Manager - a lingering process overwrites prefs.js on exit).
2. Open your profile folder:
   • installed Firefox: about:support -> Open Profile Folder
   • portable Firefox: <FirefoxPortable>\Data\profile
3. Create a chrome folder there if it does not exist, and copy userChrome.css and userContent.css into it.
4. Copy user.js into the profile root, next to prefs.js.
5. Optional - toolbar layout (hides the sidebar, downloads, account and tab-manager buttons):
   • fresh profile, never launched: rename prefs-seed.js to prefs.js and drop it in
   • profile already in use: open its prefs.js with Firefox closed and paste in the lines from prefs-seed.js
6. Start Firefox.

user.js is re-applied on every launch, which is what you want for theme prefs. The toolbar layout deliberately lives in prefs.js instead, so Customize Toolbar keeps working.

What the prefs do

Pref	Why
toolkit.legacyUserProfileCustomizations.stylesheets	Without it Firefox ignores userChrome.css entirely
widget.windows.mica	Turns on the Windows 11 Mica backdrop
ui.systemUsesDarkTheme	Dark chrome without a theme add-on - an add-on sets lwtheme on the root, which switches the theme's own colour rules off
layout.css.prefers-color-scheme.content-override	Keeps websites on the light scheme while the UI stays dark
browser.tabs.allow_transparent_browser + browser.display.background_color	Transparent about:blank and empty tabs
browser.toolbars.bookmarks.visibility	The hover-reveal bookmarks bar needs the toolbar set to always

Configuration

All of FoxOne's variables still apply - see its customisation guide. The transparency-specific tweaks live in a clearly marked LOCAL OVERRIDES block at the end of userChrome.css:

What	Where
Delay before the bookmarks bar opens	--uc-dynamic-bookmarks-hover-delay in the :root block
Bookmarks bar slide speed	220ms in override 3
Height of the resting sliver	2px in override 3
Mica backdrop style	widget.windows.mica.toplevel-backdrop - 2 = Mica, 4 = Mica Alt

Delete the whole LOCAL OVERRIDES block to fall back to stock FoxOne.

Other platforms

Mica is a Windows 11 feature. On Windows 10, macOS or Linux the transparency rules stay active with nothing painting behind them, and the toolbar renders black. Delete override 2 (Windows 11 Mica) from the end of userChrome.css on those systems.

FAQ

Does the page content blur behind the toolbar? No, and it cannot. Mica paints the window backdrop only where Firefox has not drawn content, and it samples the desktop wallpaper rather than the window underneath. Anything overlaying the page - bookmarks bar, URL-bar dropdown - therefore stays opaque by design.

The toolbar is black instead of transparent. Either you are not on Windows 11, or transparency effects are off in Windows settings, or widget.windows.mica did not apply. The rules make the toolbar transparent; if nothing paints behind it, you see black.

The toolbar text is dark and unreadable. The dark colour scheme did not apply. Check that ui.systemUsesDarkTheme is 1 and that no theme add-on is active in about:addons -> Themes. The built-in Dark theme sets lwtheme and disables the colour rules.

Do websites turn dark? No. layout.css.prefers-color-scheme.content-override keeps page content on the light scheme while the browser UI stays dark.

Credits and license

• FoxOne by Firnschnee - the base theme (MIT)
• FoxOne itself is inspired by Cascade and LittleFox

Released under the MIT License.

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