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Mozilla

Senior Contract Software Engineer (JavaScript) – Firefox

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Remote from
UK
Salary
Undisclosed
Department
QA & Testing
Employment
Contract
Experience
Senior
Published
Apply before
28 Nov 2026
Listing views
19
Application actions
2
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AI Summary

The role, at a glance.

Mozilla is seeking a senior contract engineer to improve test coverage and correctness in Firefox Gecko's Session History area. The role centers on authoring Web Platform Tests and Gecko mochitests, diagnosing failures, and resolving underlying browser-engine defects. The engineer will work through public bug tracking, code review, and upstream landing with Gecko module owners. Strong JavaScript, C++, browser-engine experience, and debugging ability in a large mature codebase are essential.

Role DNA

A quick view of the complexity, pace, ownership and collaboration implied by the job description.

Job Complexity

5/5
EasyHard

Pace & Pressure

4/5
RelaxedFast-paced

Autonomy Level

4/5
GuidedFull ownership

Communication Load

4/5
IndependentCollaborative
AI insightThis is a highly specialized browser-engine role requiring productive work in Gecko, C++, JavaScript test infrastructure, and web standards. Engineers must independently investigate complex correctness issues while collaborating effectively through open-source review processes.

Salary analysis

Estimated compensation compared with the broader US market for similar roles.

Estimated job median (per year)Market rate
$150,000
US market range (per year)$125k–$180k
AI insightThe yearly pay period is inferred from the job context; it was not explicitly stated by the employer. No employer compensation was disclosed. The displayed annual USD estimate is an AI-generated market estimate for a senior contract browser-engine and test-automation specialist; the yearly period is inferred from the senior full-time-equivalent engineering context.

Core skills

Skills and capabilities most closely associated with this opportunity.

JavaScriptC++Browser engine developmentFirefox GeckoWeb Platform TestsMochitestAutomated testingSoftware debuggingWeb standardsCode reviewOpen-source developmentTest coverage analysis
Sample interview questions
How would you investigate a failing Session History test in Gecko?

I would first reproduce the failure locally and isolate it with the smallest relevant test case. I would inspect test logs, assertions, and the affected Session History code path, use debugging tools to identify state transitions, then determine whether the test expectation or implementation is incorrect before submitting a focused fix with regression coverage.

What is your approach to writing durable Web Platform Tests?

I start from the relevant web specification and define observable behavior that remains implementation-independent. I keep tests minimal, deterministic, and isolated, cover edge cases and asynchronous behavior explicitly, and verify that the assertions can distinguish a correct implementation from the defect being addressed.

Describe how JavaScript and C++ work together in a browser-engine testing role.

JavaScript is commonly used to exercise browser-facing APIs and build automated test scenarios, while C++ is used to understand and correct underlying engine behavior. Effective work requires tracing a browser API action from the test through bindings and internal components to locate the true source of incorrect behavior.

How do you handle code review in an open-source project with module owners?

I provide a clear problem statement, focused patch, test evidence, and rationale tied to expected behavior or standards. I treat review feedback as an opportunity to improve correctness and maintainability, respond promptly to requested changes, and keep the patch scope narrow enough to be safely reviewed and landed.

How would you prioritize additional test coverage for Session History?

I would review existing coverage, historical bugs, specification requirements, and code paths with complex state handling. I would prioritize areas where regressions are likely or where behavior is security- and user-impacting, then add targeted tests that validate both common navigation flows and boundary conditions.

This analysis is generated from the job description. Salary estimates, role characteristics and sample answers are guidance, not employer-provided facts.
Opportunity details

About this 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.

Apply now >

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