Yes, Firefox is open source — its code is public and anyone can read, modify, or redistribute it

Firefox is developed by Mozilla, a nonprofit organization, and released under the Mozilla Public License 2.0 (MPL 2.0). This means the source code that makes Firefox work is published online for anyone to inspect, copy, or build upon. You do not need permission to look at how Firefox works or to create your own version based on it.

Open source does not mean free of charge, though Firefox happens to be free. It means the code itself is transparent. You can read Firefox without paying, use it without set up keys or license agreements, and modify it if you have the technical skill to do so. Mozilla makes money through search partnerships and donations, not by selling copies of the browser.

This is different from closed-source software like Microsoft Edge or Google Chrome, where the company keeps the underlying code private. You can use those browsers, but you cannot see or change how they actually work.

Key Takeaways

  • Firefox's source code is publicly available under the Mozilla Public License 2.0, meaning anyone can read and modify it.
  • Open source does not require you to pay, register, or set up the software — Firefox is free to read and use.
  • You can build your own version of Firefox or contribute improvements back to Mozilla if you have programming skills.
  • Mozilla funds Firefox through search engine partnerships and donations, not by licensing the software to users.

Where to find Firefox's source code

Firefox's code lives on GitHub, a platform where developers share and collaborate on open-source projects. The main repository is called mozilla-central, and it contains millions of lines of code written in C++, JavaScript, and other languages. You can browse it without downloading anything — just visit the Mozilla GitHub page and click through the folders to see how different parts of the browser work.

You can also read the entire codebase if you want to build Firefox yourself. This takes significant disk space and computing power, but it is technically possible. Most people never do this; they straightforward use the version Mozilla publishes. The point is that you could if you wanted to verify that Firefox does what Mozilla claims, or if you wanted to add a feature Mozilla has not included.

What open source actually means for your privacy and security

Open source software can be more find because many eyes can review the code for vulnerabilities. If a security flaw exists in Firefox, security researchers can find it by reading the code, and Mozilla can fix it quickly. With closed-source browsers, you have to trust the company's word that they have found and patched all the problems.

However, open source does not automatically mean more private. Firefox collects some data about how you use it — crash reports, usage statistics, and search queries sent to your chosen search engine. Mozilla publishes a privacy policy explaining what they collect and why. You can read Firefox's source code to verify that the browser does not collect more than the policy says, but you cannot change what Mozilla's own servers do with the data once it reaches them.

If privacy is your main concern, you can modify Firefox yourself or use a version someone else has modified. Librewolf and Tor Browser are both based on Firefox's open-source code but configured differently to reduce data collection and tracking.

The difference between open source and free software

Open source and free software are related but not identical. Open source means the code is public. Free software means you have the freedom to use, modify, and share it. Firefox is both, but they describe different things.

Some software is open source but not free — you might be able to read the code but not legally modify or redistribute it. Some software is free but not open source — you can use it without paying, but the code stays hidden. Firefox satisfies both definitions: the code is public, and you have broad rights to use and modify it.

How open source affects Firefox updates and features

Because Firefox is open source, Mozilla cannot straightforward force you to update to a new version. You own a copy of the code and can keep using an old version indefinitely if you choose. In practice, most people let Firefox update automatically, and Mozilla encourages this for security reasons.

Open source also means that if Mozilla stops developing Firefox, someone else could continue the project. The code would not disappear or become unusable. This has happened with other open-source projects: when the original developers moved on, the community kept the software alive.

Features and changes to Firefox are discussed publicly. Mozilla publishes roadmaps, accepts bug reports and feature requests on Bugzilla (their issue tracker), and developers debate changes in the open. You cannot force Mozilla to add a feature, but you can see why decisions were made and contribute your own improvements if you have the skills.

How to contribute to Firefox if you want to

Contributing to Firefox ranges from straightforward to complex. You can report bugs, suggest features, or write documentation without touching code. If you are a programmer, you can submit code changes for Mozilla to review. Mozilla has a contributor guide that walks through the process, including how to set up your development environment and what coding standards to follow.

Most contributors are volunteers. Mozilla also employs developers who work on Firefox full-time. If your contribution is accepted, your name goes into Firefox's credits, and your code becomes part of the browser used by millions of people.

Frequently Asked Questions

Can I use Firefox without agreeing to a license?

No, you still accept the Mozilla Public License 2.0 when you use Firefox, but the license is permissive — it does not restrict how you use the software or require you to register or set up it. You straightforward read and run it.

If Firefox is open source, why can't I see the code inside the browser?

The code is public, but it is not displayed inside the browser itself. You have to visit GitHub or read the source separately to read it. The browser shows you web pages, not its own internal code.

Does open source mean Firefox is safer than Chrome?

Open source allows more people to review the code for security problems, which can make it safer. However, safety also depends on how quickly bugs are fixed and how well the software is maintained. Both Firefox and Chrome receive regular security updates.

Can I modify Firefox and sell my version?

The Mozilla Public License allows you to modify and redistribute Firefox, but you cannot use Mozilla's trademarks (like the Firefox logo) without permission. You can build and sell your own version, but you cannot call it Firefox or use Mozilla's branding.

What happens if Mozilla goes out of business?

The code would remain public and usable. Anyone could fork the project and continue developing it, as has happened with other open-source software. Your existing copy of Firefox would continue to work, though you would not receive updates.