Contract Software Engineer - DOM Core Firefox/Gecko

Coinscapture; United Kingdom; Remote; GBP 60,000 - 80,000; Full time

Job summary

Coinscapture is seeking a Contract Software Engineer to contribute to the Firefox DOM Core/Gecko testing effort on-site in the United Kingdom. You will write Web-Platform tests, contribute Gecko mochitests, and fix issues to improve Gecko correctness.

The role focuses on test automation, debugging, and collaboration through public bugs and upstream review, with prior Gecko experience preferred.

Qualifications

• Substantial experience in a browser-engine codebase or 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 large codebase.
• Comfort working in the open: public bugs, code review, and upstream landing.

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.

Skills

JavaScript

C++

Browser engine

Test automation

Debugging

Open source

Tools

Gecko

Web Platform Tests

Job description

# Contract Software Engineer - DOM Core Firefox/GeckoMozillaOn-siteUnited KingdomSoftware EngineeringPosted Sep 24, 2026The 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.