Who should use Custom CSS?

Custom CSS is a consumer product built for firefox users and web developers familiar with CSS.

Yes — Custom CSS is free to use.

What platforms does Custom CSS run on?

Custom CSS runs on the web and embeddable surfaces.

Is Custom CSS still maintained?

Unverified. Custom CSS has not been re-checked since it entered the index, so there is no finding either way — and only a positive finding would say otherwise.

Who makes Custom CSS?

Custom CSS is developed by Kenny Ooi.

When did Custom CSS launch?

Custom CSS first shipped in 2022.

Back to the index

Custom CSS

Name: Custom CSS
Author: Kenny Ooi

mozilla.orgBrowser DevTools & extensions

Custom CSS is a Firefox extension that lets users inject their own CSS into websites and override default styling. It supports adjustments such as font size, font color, and other visual presentation changes for users familiar with CSS.

FreeWebEmbeddable

Visit mozilla.org

Overview

6 features

Custom CSS is a Browser DevTools & extensions project. It focuses on changing website appearance without modifying site source code or relying on built-in styling options. It is built as a consumer product for firefox users and web developers familiar with CSS. Custom CSS is free to use. Custom CSS is available on the web and embeddable surfaces.

Kenny Ooi builds and maintains Custom CSS, and it first shipped in 2022. Key capabilities include CSS injection, style overrides, and font customization. The interface is available in 32 languages, including Czech, German, and Greek.

Summary written by a language model from the project’s public pages.

CSS injection
Style overrides
Font customization
Website styling
Browser tabs access
All-sites access

Built with & integrations

Runs on

BrowserEmbeddable

Trust & compliance

Verified signals

HTTPSPrivacy Policy Terms of ServiceFree tierMulti-languageStore-verified publisher

Legal

Indexing history

What PulseGate has recorded for this listing

Indexed9 Sept · 08:14 UTC
Custom CSS verified via Firefox Add-ons
Source: Firefox Add-ons · Open

Frequently asked questions about Custom CSS

What does Custom CSS do?: Custom CSS focuses on changing website appearance without modifying site source code or relying on built-in styling options. It is catalogued under Browser DevTools & extensions on PulseGate.
Who should use Custom CSS?: Custom CSS is a consumer product built for firefox users and web developers familiar with CSS.
Is Custom CSS free?: Yes — Custom CSS is free to use.
What platforms does Custom CSS run on?: Custom CSS runs on the web and embeddable surfaces.
Is Custom CSS still maintained?: Unverified. Custom CSS has not been re-checked since it entered the index, so there is no finding either way — and only a positive finding would say otherwise.
Who makes Custom CSS?: Custom CSS is developed by Kenny Ooi.
When did Custom CSS launch?: Custom CSS first shipped in 2022.
What languages does Custom CSS support?: The interface is available in 32 languages, including Czech, German, and Greek.

At a glance

Pricing: Free
Platforms: Web
Built for: Firefox users and web developers familiar with CSS
Model: B2C
Solves: Changing website appearance without modifying site source code or relying on built-in styling options.

Registered as

Firefox Add-ons: qb-custom-css

Developer

Kenny Ooi

Index record

Indexed: 9 Sept 2026
Lifecycle: Alive
Last seen: 9 Sept 2026

Identity audit (13)

Slug: custom-css-mozilla-org
Lifecycle state recorded: 9 Sept 2026
Verification state: Indexed for public listing
Listing state: Listed: yes
Index status: Included in index
Latest evidence snapshot: 9 Sept 2026
Timeline basis: Indexed-at chronology. This listing's first-seen date was written by the catalog backfill, not observed here, so it is not treated as a sighting.
Name from: Derived from the project's own page and URL.
Category from: Assigned by a language model.
Summary from: Written by a language model from public pages.
Languages from: Detected by a language model, checked against the page's own declaration.
Last updated: 9 Sept 2026
Canonical URL: https://addons.mozilla.org/en-US/firefox/addon/qb-custom-css

Ship this? Send a correction — no account, and you get a link to follow it.

Similar projects

Closest matches by what these projects do