> Markdown version of [/jobs/ext/3322786-contract-software-engineer-dom-core-firefox-gecko](https://www.wearedevelopers.com/jobs/ext/3322786-contract-software-engineer-dom-core-firefox-gecko). Every page supports `.md` or `Accept: text/markdown`. Links point to the HTML versions so they work for humans too. Agent guide: [/agents.md](https://www.wearedevelopers.com/agents.md).

---

# Contract Software Engineer - DOM Core Firefox/Gecko

- **Company:** Mozilla
- **Location:** London, UK
- **Contract:** Permanent contract
- **Skills:** JavaScript (Programming Language), Automation of Tests, C++ (Programming Language), Code Review, Firefox, Gecko (Software), Software Engineering, Web Platforms, Web Standards, Codebase
- **Published:** September 22, 2026
- **Apply:** https://www.indeed.com/viewjob?jk=358d0bffb2963532

## About the Role

* Substantial experience in a browser-engine codebase or in automating for a browser-engine, ideally Firefox/Gecko.
 * Solid Javascript and applying that language skill to write automated tests.
 * Proficiency in and experience developing software in C++.
 * Strong debugging skills in a mature larget codebase.
 * Comfort working in the open: public bugs, code review, and upstream landing.

Preferred

 * Prior contributions to Gecko or another major browser engine.
 * Familiarity with web standards and Web Platform Tests.

## Description

You will help the Firefox DOM Core team improve the health of the Session History area. You will write tests to improve the coverage; you will fix tests and/or problems to improve the Gecko correctness., * Write Web-Platform tests to improve the coverage, and Gecko mochitests as needed

 * Fix the problems and pass the tests to improve the Gecko correctness
 * Work through code review with the relevant Gecko module owners and iterate to landing