Senior Software Engineer, WebExtensions

About the Role: Mozilla Corporation is the non-profit-backed technology company shaping the next 25 years of the internet. We are looking for a Senior Software Engineer to join the WebExtensions team, which powers the add-ons ecosystem in Firefox. You will work deep within the Gecko platform to evolve extension capabilities, balancing developer power with user trust, safety, and performance.

What You'll Do

• Implement and maintain WebExtensions APIs and Addon Management interfaces within Firefox.
• Debug and fix complex issues affecting browser extensions, ensuring reliable performance for millions of users.
• Collaborate with product managers, QA, and the AMO (Add-ons Mozilla) engineering team on end-to-end feature delivery.
• Contribute to Mozilla-owned extensions and system add-ons while participating in open-source workflows (GitHub/Bugzilla).
• Support extension developers by diagnosing technical issues and improving documentation and tooling.

What You Bring

• 4+ years of professional experience in software development.
• Strong proficiency in JavaScript, DOM APIs, HTML, and CSS.
• Familiarity with modern browser architecture, debugging tools, and API design.
• Ability to navigate, understand, and contribute to large-scale, complex codebases.
• Strong communication skills and experience collaborating in distributed teams.
• Bonus: Experience with Firefox internals (C++ or Rust), GeckoView, web security principles, or open-source browser development.

Benefits

• Performance-based bonus plans and rich medical, dental, and vision coverage.
• Generous retirement contributions with 100% immediate vesting.
• Remote-first environment with home office stipends, wellness days, and professional development budgets.
• Paid parental leave, birthday leave, and country-specific holidays.
• Join an mission-driven organization that values open-source software and internet accessibility for all.