Fennec

Fennec is a minimal Firefox setup built with userChrome.css, designed around vertical tabs, zen mode, and keyboard-driven browsing. One CSS file, no fork, no build — the sidebar-first workflow of Zen Browser without leaving Firefox.

Sidebar Open	Zen Mode

Features

🔗 Enhanced Sideberry Integration - Urlbar inside the sidebar-box, tracks sidebar width, and expands when focused

🧘 Zen Mode - Toggling the sidebar hides the UI, maximizing screen space and aiding focus when tiled or maximized

✨ Minimal Chrome - Only essential objects exposed, coherent with a keyboard driven UX

🛠️ Community Minded - Clean code and detailed docs to support customization and contribution

🎨 Theme Support - System themes (light-dark) supported. User created Firefox themes are also supported.

Installation

Please see security considerations before installing

1. Install the Sideberry Extension

Install Sideberry from Firefox Add-ons.

2. Install CSS

Choose one of the two methods below:

Option A: Automated (recommended)

The script does the following:

• Backs up your existing chrome folder (if any) to chrome.bak.<timestamp>
• Copies Fennec's chrome/ files into your Firefox profile
• Writes prefs to user.js: disables vertical tabs, disables the sidebar revamp, enables custom stylesheets

To uninstall: delete the chrome folder and remove the Fennec lines from user.js in your profile directory (or delete user.js entirely if Fennec created it). Your backup is in chrome.bak.*.

macOS / Linux:

curl -fsSL https://raw.githubusercontent.com/tompassarelli/fennec/main/install.sh | bash

Windows (PowerShell):

irm https://raw.githubusercontent.com/tompassarelli/fennec/main/install.ps1 | iex

Option B: Manual

Enable required Firefox settings:

Note: only toolkit.legacyUserProfileCustomizations.stylesheets requires about:config. The rest are defaults historically and can also be changed in Settings.

1. Go to about:config in the address bar
2. Set toolkit.legacyUserProfileCustomizations.stylesheets to true
3. Set sidebar.verticalTabs to false (or turn on Horizontal tabs in Settings)
4. Set sidebar.revamp to false (or turn off Show Sidebar in Settings)

Locate your Firefox profile directory:

1. Go to about:support in the address bar
2. Under "Application Basics", click Open Profile Folder
   • Flatpak users: the profile directory is at ~/.var/app/org.mozilla.firefox/.mozilla/firefox/<profile>

Copy the CSS files:

1. Inside the profile folder, create a chrome directory if it doesn't already exist
2. Copy userChrome.css from this repo's chrome/ folder into that chrome directory
3. Copy autohide.css into the same chrome directory (needed if you want autohide)

3. Restart Firefox

• Note: if the sidebar is invisible, you might have it toggled off. Try Ctrl+H to toggle history, then activate the Sideberry tabs menu from there by clicking on the extension icon.

Alternative: Nix / Home Manager

Fennec can also be installed declaratively via a Home Manager module — this handles CSS, prefs, and Sideberry in one step.

1. Add fennec to your flake inputs:

inputs.fennec.url = "github:tompassarelli/fennec";

2. Import the module in your Home Manager config:

imports = [ inputs.fennec.homeManagerModules.default ];

3. Enable it:

programs.fennec = {
  enable = true;
  profile = "your-profile-name";  # optional, defaults to "default-release"
  autohide = false;               # optional
};

4. Rebuild with nixos-rebuild switch or home-manager switch

Note: Sideberry is installed automatically via NUR. Ensure NUR is in your flake inputs and overlays. Set sideberry = false if you manage extensions separately.

Optional Features

Autohide (off by default)

Sidebar must be enabled (not toggled off). When enabled, the drawer auto-collapses when the mouse leaves and reappears when hovering the left edge of the window.

To enable:

1. Ensure autohide.css is in the same chrome directory as userChrome.css (see installation step 2)
2. Uncomment @import url("autohide.css"); in userChrome.css
3. Restart Firefox

Recommended Extensions

• Vimium - Keyboard-driven navigation that complements the minimal, distraction-free interface

Help

If something isn't working, check open issues or file a new one.

Security Considerations

• The install guide directs users to download Firefox extensions. Firefox extensions can introduce security vulnerabilities and/or take direct hostile actions against users.
• Zen Mode hides the UI which obviously suppresses security signals like padlock warnings. In appreciation of this concern, Fennec will still attempt to surface a custom HTTP Not Secure security warning prepended to page content as a header alert. Not a solution against phishing and other attacks/vulnerabilities, only toggle the UI after the page has been verified as secure and trustworthy.
• Use at your own risk - The author is not liable for any security issues, data breaches, or other damages of usage of this repository or mentioned extensions.
• You are responsible for verifying the security of websites, code, and extensions used
• Always keep Firefox updated

By using this theme and mentioned Firefox extensions, you acknowledge these risks and agree that the author bears no responsibility for any consequences.