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Mozilla

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

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Remote from
France, Netherlands, Poland+3 more, Finland, Belgium, Denmark
Salary
Undisclosed
Department
QA & Testing
Employment
Contract
Experience
Senior
Published
Apply before
26 Nov 2026
Listing views
29
Application actions
1
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AI Summary

The role, at a glance.

Mozilla is seeking a senior contract engineer to improve Firefox Gecko Session History quality through automated testing and targeted correctness fixes. The work centers on writing Web Platform Tests and Gecko mochitests, diagnosing failures in a mature browser-engine codebase, and landing reviewed changes upstream. Candidates need strong C++ and JavaScript expertise, browser-engine or browser automation experience, and confidence working transparently through public bugs and code review. This is a remote contract role open to candidates in Belgium, Denmark, Finland, France, the Netherlands, and Poland.

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

5/5
GuidedFull ownership

Communication Load

4/5
IndependentCollaborative
AI insightThe role requires deep familiarity with complex browser-engine internals, C++ debugging, web standards, and test infrastructure. Engineers must independently investigate correctness issues and navigate rigorous public upstream review processes.

Salary analysis

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

Estimated job median (per year)Highly competitive
$150,000
US market range (per year)$120k–$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 annual USD figures are AI-estimated market guidance for a senior contract browser-engine engineer and use an inferred yearly period for comparison purposes.

Core skills

Skills and capabilities most closely associated with this opportunity.

C++JavaScriptWeb Platform TestsGecko mochitestsBrowser engine developmentFirefox GeckoAutomated testingSoftware debuggingWeb standardsCode reviewSession HistoryUpstream contribution
Sample interview questions
How would you investigate and fix a Session History correctness failure in Gecko?

I would first create a minimal reproducible case and determine whether the failure is in DOM behavior, history state handling, or test assumptions. I would trace the relevant C++ and JavaScript execution paths, add focused logging or assertions where appropriate, implement the smallest safe fix, and validate it with targeted mochitests and Web Platform Tests.

What is your approach to writing effective Web Platform Tests?

I start from the applicable web specification and identify observable interoperability behavior. I keep tests isolated and deterministic, use clear setup and cleanup, cover edge cases and regressions, and ensure the test validates standards behavior rather than a browser-specific implementation detail.

Describe how you debug a failure in a large C++ codebase that you did not write.

I begin by narrowing the failure with a reliable reproduction and reviewing recent related changes. I map the relevant call flow, inspect state transitions with a debugger and targeted instrumentation, form and test hypotheses incrementally, then add a regression test that would fail without the fix.

How do you handle upstream code review when reviewers request changes that alter your proposed solution?

I treat review as a collaborative design process. I clarify the reviewer’s concern, assess alternatives against correctness, maintainability, and project conventions, update the patch and tests accordingly, and explain the reasoning in the review thread so the final change is easy to evaluate.

How would you decide whether a regression belongs in a mochitest or a Web Platform Test?

I would use a Web Platform Test when the behavior is standardized and interoperability-relevant. I would use a mochitest when validating Gecko-specific internals, integration behavior, or conditions that cannot be expressed portably through the web platform test suite; in some cases, I would add both.

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.

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