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Mozilla

Senior Contract C++ / JavaScript Software Engineer Firefox (Browser Engine)

Remote France
Level
Senior
Work type
Remote · France only

Not stated in the posting: salary, visa.

Open. First seen 20 October 2026.

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About the role

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.

The team: Firefox HIRES STEADILY

This role is one of 62 open in Firefox. 35 were posted in the last 30 days and the oldest in February: a team that hires all year rather than one being built.

WHAT THE TEAM WORKS WITH, ACROSS ITS POSTINGS

C++ (37 of 62), JavaScript (37 of 62), Rust (32 of 62), Python (29 of 62), CI/CD (18 of 62), TypeScript (18 of 62), CSS (17 of 62)

OTHER ROLES ON THIS TEAM

From the team and dates on Mozilla's own job board.

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