# Senior Contract C++ / JavaScript Software Engineer (Browser Engine) at Mozilla

> Open job at Mozilla. Workplace: Remote (France).

Source: France Dev Jobs, https://francedevjobs.com/jobs/mozilla-contract-software-engineer-dom-core-firefox-gecko-44prns9
Posted 22 September 2026. Last checked on the employer's career site: 10 November 2026.

## Facts

- Company: [Mozilla](https://francedevjobs.com/companies/mozilla)
- Location: Remote (France)
- Workplace: Remote (France)
- Employment type: Contract
- French: Not required
- Skills: C++, JavaScript
- Not mentioned in the job post: salary, visa sponsorship, relocation, where you can apply from

## Job description

### The role

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.

### Responsibilities

- 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

### Definition of done

Improved Session History test coverage and code correctness in Gecko, measured by passed test cases, allowing the team’s senior engineers to focus on security, performance and strategic projects.

### Required qualifications

- 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.

## How to apply

Apply on the job page, https://francedevjobs.com/jobs/mozilla-contract-software-engineer-dom-core-firefox-gecko-44prns9: its Apply button opens the employer's own application page.

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- [Staff Software Engineer, Tools & Systems](https://francedevjobs.com/jobs/mozilla-staff-site-reliability-engineer-tools-systems-vka0jmk): Mozilla, Remote (France). Salary: €65k–87k / year. Posted 22 October 2026.
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- [Senior Contract C++ / JavaScript Software Engineer Firefox (Browser Engine)](https://francedevjobs.com/jobs/mozilla-senior-contract-c-javascript-software-engineer-firefox-brows-uk1hpvo): Mozilla, Remote (France). Posted 20 October 2026.
- [Senior Software Engineer, Firefox Security](https://francedevjobs.com/jobs/mozilla-senior-software-engineer-firefox-security-5wyk477): Mozilla, Remote (France). Salary: €53k–71k / year. Posted 15 October 2026.