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Senior Software Engineer, Firefox Security

Mozilla· Firefox
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Hybrid (Spain)€58k - €77kfull-timePosted 6d ago

About the role

Senior software engineer focusing on designing, building, and securing Firefox across platforms, with emphasis on enterprise security features, architecture, and performance in a multi-language codebase.

Responsibilities

  • 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 by improving 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.

Qualifications

  • 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.

Skills

Required:C/C++Security fundamentalsDebugging of multithreaded appsWeb technologies
Nice to have:Rust

Benefits

  • Generous performance-based bonus plans to all eligible employees
  • Generous retirement contributions with 100% immediate vesting
  • Quarterly all-company wellness days
  • 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)
Last updated Oct 21, 2026. Interested? Apply on the company site.

About Mozilla

Internet for people, not profit

Mozilla is the not-for-profit behind the lightning fast Firefox browser. We put people over profit to give everyone more power online.

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