About this role.
Mozilla is seeking a contract software engineer to improve Session History quality in the Firefox/Gecko DOM Core area. The work centers on creating Web Platform Tests and Gecko mochitests, diagnosing failures, and correcting C++ and JavaScript code issues. The engineer will work through public bug trackers, module-owner code reviews, and upstream landing processes. Success is measured by stronger test coverage and passed tests that improve Gecko correctness and free senior engineers for security, performance, and strategic work.
Role DNA
A quick view of the complexity, pace, ownership and collaboration implied by the job description.
Job Complexity
5/5Pace & Pressure
4/5Autonomy Level
4/5Communication Load
4/5Salary analysis
Estimated compensation compared with the broader US market for similar roles.
Core skills
Skills and capabilities most closely associated with this opportunity.
Sample interview questions
I would first reproduce the failure locally and in CI-like configurations, then isolate timing, process, platform, and state dependencies. I would inspect logs and relevant Session History transitions, reduce the test to a minimal reproducer, determine whether the product or test assumption is incorrect, and submit a focused patch with reliable regression coverage.
Web Platform Tests validate interoperable web-platform behavior and should be preferred when the behavior is standards-facing and portable across engines. Mochitests are appropriate for Gecko-specific implementation details, privileged APIs, or test infrastructure needs that cannot be expressed as a WPT. I would choose the narrowest test type that accurately protects the intended behavior.
I begin by mapping the affected execution path and ownership boundaries, then use targeted logging, assertions, debuggers, and existing tests to validate hypotheses. I avoid broad speculative changes, identify the underlying invariant violation, and add a regression test that fails before the fix and passes afterward.
I would make state setup and cleanup explicit, avoid arbitrary delays, wait on deterministic events or promises, and minimize reliance on environmental assumptions. I would run the test repeatedly under relevant configurations and review it for hidden ordering, timing, or cross-test-state dependencies.
I provide a concise problem statement, minimal patch series, clear test evidence, and documentation of tradeoffs. I respond constructively to reviewer feedback, incorporate module-owner guidance, keep bug discussions transparent, and ensure the final change is scoped, tested, and ready for upstream maintenance.
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.
Annual salary information is not provided for this position. Explore salary ranges for similar roles in our Salary Directory ›
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