Engineering service
Custom Firefox browser development
We build Firefox-based browsers and managed Firefox deployments. We use enterprise policies and preferences where they are enough, patch Gecko and the Firefox UI where they are not, and keep the result current with each Firefox or ESR release.
- 11+ years of experience
- 300+ projects delivered
- Remote team, clients worldwide
- NDA on request
Overview
What it is and when it fits
Firefox is built on the Gecko engine and the SpiderMonkey JavaScript engine, with a growing share of components written in Rust. Its source moved from Mercurial (mozilla-central) to Git on GitHub in 2025, and it is built with the mach tool. It is a different codebase from Chromium, with its own extension system and policy model.
Many Firefox customizations need no fork. Enterprise policies, preferences and autoconfig cover a lot of ground. We start there, and move to source patches only for what configuration cannot reach.
Who this is for
- Organizations that need Firefox configured and locked down across many machines.
- Teams building a privacy-focused or branded browser on Gecko instead of Chromium.
- Companies that want to know whether they need a fork at all before committing to one.
- Products that already ship a Firefox-based build and need help keeping it current.
How it works
Architecture at a glance
Anatomy of a customized Firefox
Your build
Your name and icons, default settings, search, new tab and onboarding.
Configuration and patches
Policies and preferences first; source patches only where needed.
Firefox and Gecko
Gecko engine, SpiderMonkey, the browser UI and Rust components from the release or ESR branch.
Build and release
- Windows
- macOS
- Linux
mach builds, CI, installers, signing and update delivery.
- Your productWhat your users see
- Our engineeringWhat we build and maintain
- UpstreamThird-party code we build on
- Platforms and releaseBuild, sign and deliver
What we offer
What we build on Firefox
Firefox build pipeline
mach and mozconfig setup so your team can produce reproducible builds from the Firefox release or ESR branch on Windows, macOS and Linux.
Enterprise policies and autoconfig
Policy files and autoconfig scripts that lock settings, manage certificates and extensions, and control update behavior without touching source code.
Privacy defaults and Mozilla service calls
Telemetry, studies and sponsored content off by default, and a review of every connection the browser makes to Mozilla services.
Networking changes in Necko
Changes to Necko, Firefox's networking layer, for proxy handling, certificate behavior or request routing that policies cannot reach.
Rendering changes in Gecko
Work in Stylo, Firefox's Rust-based CSS engine, and the rest of the Gecko rendering path when behavior has to change, not only styling.
ESR tracking
Builds that follow Firefox's Extended Support Release, for organizations that want to move less often than the four-week release train.
Deliverables
What you get
- 01
Customization assessment
A written breakdown of which of your requirements need a policy, an extension or a source patch.
- 02
Policy and preference set
A tested policies.json and preference configuration for your deployment, including autoconfig where it applies.
- 03
Rebranded build
Your own name and icons in place of Mozilla's trademarks, plus home and new-tab pages, search engines and first-run flow.
- 04
Source patches, if required
A small patch series on a named Firefox version for anything configuration cannot do.
- 05
Build and release pipeline
Reproducible mach builds from the Firefox release or ESR branch, with CI, packaging, signing and update delivery for the platforms you choose.
- 06
Upgrade runbook
Steps for moving your build onto new Firefox releases, or onto the next ESR if that suits your needs.
Compare
Three ways to customize Firefox
These are ordered from lightest to heaviest. Most projects should stop at the first option that meets the requirement.
| Method | What it can do | Effort to maintain | Use when |
|---|---|---|---|
| Enterprise policies and preferences | Set defaults, lock settings, control updates, block features, manage extensions and certificates | Low | You want a managed, configured Firefox and stock Firefox binaries are acceptable. |
| Extension or theme | Change page behavior, add UI within extension APIs, restyle the browser | Low to medium | Your feature fits the WebExtensions APIs and can be installed into existing Firefox. |
| Source patches (custom build) | Change browser internals, engine behavior, built-in UI and default features | High | You need behavior that policies and extensions cannot provide, or you ship your own branded browser. |
Why it matters
What this gives you
Configuration before code
Many Firefox requirements are met with policies.json and preferences, so you avoid the ongoing cost of a source patch unless you need one.
An engine independent of Google's roadmap
Gecko is the main browser engine not built on Chromium or WebKit, so your product does not depend on decisions made for Chrome.
Extensions Chrome no longer allows
Firefox still supports Manifest V2 and blocking webRequest, so ad blockers and privacy tools keep capabilities they lost in Chrome.
Longer support windows with ESR
ESR lets managed deployments take security fixes without taking new features every four weeks.
Our approach
How a Firefox project runs
Assess
We list your requirements and match each one to the lightest method that can meet it.
You get: A method-by-requirement assessment
Configure
We build and test the policy and preference set on the Firefox release channel you choose, including ESR if a longer-supported base fits your organization.
You get: A tested configuration
Patch where needed
For remaining gaps we write source patches, keeping each one small and documented.
You get: A patch series on a named version
Build and test
We set up mach builds and CI, then test your features and core browsing on each platform.
You get: Reproducible builds and test results
Release and maintain
We set up signing, packaging and updates, and document how to move to new Firefox versions.
You get: Signed builds and an upgrade runbook
When this is not the right fit
- A policy or an extension solves your problem. We will say so first, as it costs far less to maintain.
- Your product needs Chromium-only features or unchanged Chrome Web Store extensions. Start from Chromium instead.
Tech stack
Technologies we use
The engines, APIs and tools we use for Firefox-based browsers.
FirefoxGeckoSpiderMonkey
RustStylomachNeckopolicies.json
C++ESRFAQ
Common questions
Often policies, preferences and autoconfig are enough, especially for managed deployments. A fork makes sense when you need engine or UI changes they cannot make, or you ship a separately branded browser.
ESR is Firefox's Extended Support Release. A new ESR version arrives about once a year and receives security fixes for roughly a year, with a few months of overlap with the next one, so you move less often than on the regular four-week releases. It suits managed deployments; a consumer browser usually follows the regular channel.
No. They are Mozilla trademarks, so a modified build ships under your own name, and rebranding is part of the project. Firefox's code is under the Mozilla Public License 2.0, which means the source of any Firefox files you change must be available to your users.
We keep source changes as a small patch series and rebase it on new Firefox releases. The fewer patches you carry, the cheaper each upgrade is, which is why we push as much as possible into configuration.
Windows, macOS and Linux are the standard desktop targets. Firefox for Android is built on GeckoView and can be forked too; we scope it separately. On iOS, Apple requires browsers to use WebKit outside the EU and Japan, and even Firefox for iOS uses WebKit, so a Gecko-based iOS browser is not an option for most markets. Raise mobile early.
Yes. Firefox extensions use the WebExtensions APIs, which overlap heavily with Chrome's but differ in places; Firefox still supports Manifest V2 and blocking webRequest. We can build a Firefox extension on its own or alongside a customized build.
Our services
Related services
Firefox and Gecko
Plan your Firefox-based browser
Tell us what your Firefox-based product has to do. We will tell you where configuration ends, where source changes begin and what it takes to maintain.
