Senior Contract Software Engineer (JavaScript) - Firefox

Mozilla · posted Oct 20, 2026

Open to candidates in United Kingdom

Senior

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What this job asks for

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Must have

•Browser-engine codebase experience
•Automating for browser-engine
•Solid Javascript
•Automated tests
•Proficiency in C++
•Strong debugging skills
•Working in the open

Nice to have

•Contributions to Gecko
•Familiarity with web standards
•Web Platform Tests

Tools and technologies

JavaScript, C++, Gecko

Read 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