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Mozilla

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

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Remote from
Germany
Salary
Undisclosed
Department
QA & Testing
Employment
Contract
Experience
Senior
Published
Apply before
20 Nov 2026
Listing views
21
Application actions
0
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AI Summary

The role, at a glance.

This senior contract role supports Mozilla’s Firefox DOM Core team by improving the reliability of Gecko Session History. The engineer will create Web Platform Tests and Gecko mochitests, diagnose failures, and implement C++ or JavaScript fixes that improve correctness. Work is performed in the open through public bug tracking, code review, and upstream landing processes. Success is measured by stronger test coverage and passing test cases that free senior engineers to focus on security, performance, and strategic work.

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 browser-engine development, C++, JavaScript test automation, and debugging within a large mature codebase. Contributors must independently investigate complex correctness issues while navigating public reviews and upstream standards.

Salary analysis

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

Estimated job median (per year)Market rate
$165,000
US market range (per year)$140k–$210k
AI insightThe yearly pay period is inferred from the job context; it was not explicitly stated by the employer. No employer compensation was disclosed. Estimated annual market compensation is shown in USD for a senior browser-engine software engineer; the yearly cadence is inferred from the senior full-time-equivalent contract role context.

Core skills

Skills and capabilities most closely associated with this opportunity.

C++JavaScriptFirefox GeckoBrowser engine developmentWeb Platform TestsMochitestAutomated testingDebuggingCode reviewWeb standardsOpen-source development
Sample interview questions
How would you investigate an intermittent Session History failure in Gecko?

I would first reproduce the failure locally and in automation, then reduce it to the smallest reliable test case. I would inspect logs, relevant DOM and navigation state transitions, and recent changes before using targeted instrumentation to identify the incorrect assumption or race.

What is your approach to writing a valuable Web Platform Test?

I start from the applicable web-platform behavior and define an observable assertion that is portable across browsers. I keep the test narrowly scoped, deterministic, and independent of implementation details, then verify it fails before the intended fix and passes afterward.

Describe how you would decide whether a failing test needs a test fix or a product-code fix.

I would compare the test expectation with the relevant specification and existing browser behavior. If the expectation is invalid, flaky, or depends on undefined behavior, I would correct the test; if Gecko violates the required behavior, I would fix the implementation and preserve the regression test.

How do you work effectively through open-source code review?

I provide a focused patch with a clear problem statement, tests, and rationale for design choices. I address reviewer feedback promptly, explain trade-offs with evidence, and split unrelated changes so maintainers can evaluate the work efficiently.

How do C++ and JavaScript complement each other in Gecko testing work?

JavaScript is effective for expressing browser-facing behavior in Web Platform Tests and mochitests, while C++ is essential for diagnosing and correcting engine-level implementation issues. I use the test layer to capture the expected behavior and the C++ layer to repair the underlying state management or DOM logic.

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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