Suggested rewrite: Led a cross-functional initiative that improved [business outcome] by [measurable result], demonstrating experience relevant to this role...
Senior Software Engineer, Firefox Security
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- Remote from
- France
- Salary
- EUR 53k–71k / yr
- Department
- Cybersecurity
- Employment
- Full Time
- Experience
- Senior
- Published
- Apply before
- 26 Nov 2026
- Listing views
- 37
- Application actions
- 3
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The role, at a glance.
Mozilla is seeking a senior engineer to strengthen Firefox security through systems-level design, implementation, testing, and incident investigation. The role focuses on browser hardening, privilege isolation, trust boundaries, cryptographic protocols, WebPKI, and security-sensitive web APIs. The engineer will diagnose stability, memory, correctness, and performance issues in a large multithreaded, cross-platform native codebase. Strong C or C++ skills, security fundamentals, web-platform familiarity, and effective collaboration with distributed teams are central to success.
Role DNA
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Pace & Pressure
4/5Autonomy Level
4/5Communication Load
4/5Salary analysis
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Core skills
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Sample interview questions
I would first map assets, entry points, trust boundaries, privileges, and attacker capabilities. I would then prioritize isolation, least privilege, strict input validation, safe ownership and memory practices, and defense-in-depth controls, supported by threat modeling, targeted tests, fuzzing, and security review.
I begin by making the problem reproducible and collecting logs, crash data, and relevant traces. I use tools such as sanitizers, debuggers, profilers, and race detectors to narrow the fault, validate the root cause with a minimal reproduction, implement a safe fix, and add regression coverage.
I would identify what data and capabilities cross the boundary, verify authentication and authorization assumptions, minimize exposed interfaces, and ensure failures default to a safe state. I would also evaluate compatibility and performance effects, involve relevant reviewers early, and test adversarial as well as normal flows.
I would confirm the relevant standards, existing browser behavior, threat model, and interoperability requirements. The implementation should use established cryptographic primitives and libraries, avoid custom crypto, handle invalid certificates and downgrade paths safely, and include comprehensive compatibility and negative-path testing.
I tailor the explanation to the audience while clearly stating the affected component, impact, exploitability, root cause, and recommended remediation. I provide actionable evidence and reproduction details to technical owners, document risk and trade-offs, and keep stakeholders updated through validation and deployment.
About this role.
Why Mozilla?
Mozilla Corporation is the non-profit-backed technology company that has shaped the internet for the better over the last 25 years. We make pioneering brands like Firefox, the privacy-minded web browser. Now, with more than 225 million people around the world using our products each month, we’re shaping the next 25 years of technology and helping to reclaim an internet built for people, not companies. Our work focuses on diverse areas including AI, social media, security and more. And we’re doing this while never losing our focus on our core mission – to make the internet better for people.
The Mozilla Corporation is wholly owned by the non-profit 501(c) Mozilla Foundation. This means we aren’t beholden to any shareholders — only to our mission. Along with thousands of volunteer contributors and collaborators all over the world, Mozillians design, build and distribute open-source software that enables people to enjoy the internet on their terms.
What you’ll do:
- Design, build and implement security and hardening features for enterprise users: Design, implement, test, and ship cutting-edge security features that strengthen Firefox’s security posture.
- Advance Firefox’s security architecture across multiple operating systems: Contribute to and drive cutting-edge improvements to Firefox’s core security posture, by building privilege isolation and moving trust boundaries or Strengthen Firefox’s security architecture: Improve Firefox’s core security mechanisms including cryptographic protocols, WebPKI, and security-sensitive web APIs.
- Contribute to our security initiatives: Work across teams to investigate security issues, identify root causes, and deliver robust, maintainable fixes.
- Improve browser reliability and performance: Diagnose and resolve issues affecting stability, correctness, performance, and memory usage in a complex, multithreaded environment.
What you’ll bring:
- Systems programming experience and security knowledge: 3+ years of experience in low-level software engineering, strong proficiency in C and/or C++, and a solid understanding of security fundamentals.
- Debugging and problem-solving skills: Experience diagnosing and resolving issues in multithreaded, cross-platform native applications, including memory and performance challenges.
- Web platform and modern systems experience: Familiarity with web technologies (HTML, CSS, JavaScript) and interest or experience in Rust; ability to work effectively within large, complex, multi-language codebases.
- Effective collaboration and communication: Experience working with distributed teams and communicating technical ideas clearly in writing and conversation.
What you’ll get:
- Generous performance-based bonus plans to all eligible employees – we share in our success as one team
- Generous retirement contributions with 100% immediate vesting (regardless of whether you contribute)
- Quarterly all-company wellness days where everyone takes a pause together
- Country specific holidays plus a day off for your birthday
- One-time home office stipend
- Annual professional development budget
- Quarterly well-being stipend
- Considerable paid parental leave
- Employee referral bonus program
- Other benefits (life/AD&D, disability, EAP, etc. – varies by country)
About Mozilla
Mozilla exists to build the Internet as a public resource accessible to all because we believe that open and free is better than closed and controlled. When you work at Mozilla, you give yourself a chance to make a difference in the lives of Web users everywhere. And you give us a chance to make a difference in your life every single day. Join us to work on the Web as the platform and help create more opportunity and innovation for everyone online.
Commitment to diversity, equity, inclusion, and belonging
Mozilla understands that valuing diverse creative practices and forms of knowledge are crucial to and enrich the company’s core mission. We encourage applications from everyone, including members of all equity-seeking communities, such as (but certainly not limited to) women, racialized and Indigenous persons, persons with disabilities, persons of all sexual orientations, gender identities, and expressions.
We will ensure that qualified individuals with disabilities are provided reasonable accommodations to participate in the job application or interview process, to perform essential job functions, and to receive other benefits and privileges of employment, as appropriate. Please contact us at hiringaccommodation@mozilla.com to request accommodation.
We are an equal opportunity employer. We do not discriminate on the basis of race (including hairstyle and texture), religion (including religious grooming and dress practices), gender, gender identity, gender expression, color, national origin, pregnancy, ancestry, domestic partner status, disability, sexual orientation, age, genetic predisposition, medical condition, marital status, citizenship status, military or veteran status, or any other basis covered by applicable laws. Mozilla will not tolerate discrimination or harassment based on any of these characteristics or any other unlawful behavior, conduct, or purpose.
Group: D
#LI-REMOTE
Req ID: R3247
Hiring Ranges:
Remote France
€53.000—€71.000 EUR
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