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

Mozilla · Remote UK · posted Oct 20, 2026

Open to candidates in United Kingdom

senior
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What this role actually asks for

Extracted by RemoteHunt

Must have

  • •Browser-engine codebase experience
  • •Automating for a browser-engine
  • •JavaScript for automated tests
  • •C++ software development
  • •Large codebase debugging
  • •Working in the open

Nice to have

  • •Contributions to Gecko or other browser engines
  • •Web standards familiarity
  • •Web Platform Tests familiarity

Tools and technologies

C++JavaScript

Worth checking before you apply

  • ⚠contract

The full posting

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.
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