Senior Software Engineer, WebExtensions

MozillaMozilla·Remote(Remote Germany)
Software Development

WFA Digital Insight

As demand for skilled software engineers continues to rise, with a 25% increase in remote job postings in 2025, professionals with expertise in WebExtensions and digital skills are highly sought after. Mozilla, a pioneer in the tech industry, stands out for its commitment to open-source software and community-driven development. With millions of users relying on Firefox, the company's focus on safety, performance, and user trust makes this role particularly interesting. Before applying, candidates should be aware of the importance of collaboration, open-source workflows, and the need to balance developer power with user trust.

Job Description

About the Role

The Senior Software Engineer, WebExtensions, plays a critical role in shaping the future of Firefox and the web. As a member of the WebExtensions team, you will be responsible for implementing and maintaining WebExtensions APIs and Addon Management, ensuring that extensions remain safe, powerful, and reliable for millions of Firefox users. You will collaborate with experts across Mozilla, including product managers, QA, and the AMO engineering team, to deliver features end-to-end and advance cross-browser compatibility.

The WebExtensions platform powers the add-ons ecosystem in Firefox, enabling users to customize their browser while keeping safety and performance at the forefront. As a Software Engineer on the WebExtensions team, you will bring a passion for exploring complex systems and creating experiences that balance developer power with user trust.

What You Will Do

  • Implement and maintain WebExtensions APIs and Addon Management, balancing power, safety, and performance, as well as user interfaces in Firefox.
  • Debug and fix issues affecting extensions in Firefox.
  • Collaborate with product managers, QA, and the AMO engineering team to deliver features end-to-end.
  • Contribute to Mozilla-owned extensions and system add-ons.
  • Participate in open source workflows: code reviews, pull requests, and discussions in Bugzilla/GitHub.
  • Support extension developers by helping diagnose issues and improving tooling/documentation.
  • Ensure that extensions remain reliable across Firefox, focusing on safety and performance.
  • Work deep in the Gecko platform to evolve extension capabilities.
  • Build UI features in Firefox that make extensions clear and manageable for users.

What We Are Looking For

  • 4+ years of professional experience in software development.
  • Strong proficiency in JavaScript, DOM APIs, HTML, and CSS.
  • Experience with WebExtensions, add-ons, or similar technologies.
  • Ability to collaborate with experts across the organization.
  • Passion for open-source software and community-driven development.
  • Strong problem-solving skills and attention to detail.
  • Experience with agile development methodologies and version control systems.

Nice to Have

  • Experience with Gecko platform development.
  • Familiarity with cross-browser compatibility and standardization efforts.
  • Knowledge of security best practices and threat modeling.
  • Participation in open-source projects or communities.

Benefits and Perks

  • Opportunity to work on a product used by millions of people worldwide.
  • Collaborative and dynamic work environment.
  • Flexible working hours and remote work options.
  • Professional development opportunities, including training and conference attendance.
  • Access to the latest technologies and tools.
  • Competitive compensation and benefits package.
  • Opportunity to contribute to open-source software and community-driven development.

How to Stand Out

  • Ensure you have a strong understanding of WebExtensions, JavaScript, and DOM APIs before applying.
  • Be prepared to provide examples of your experience with open-source software and community-driven development.
  • Highlight your problem-solving skills and attention to detail, as these are critical for success in this role.
  • Familiarize yourself with Mozilla's products and mission, and be prepared to discuss how your skills and experience align with the company's goals.
  • Prepare to discuss your experience with agile development methodologies and version control systems, such as Git.
  • Show enthusiasm for open-source software and community-driven development, and be prepared to discuss your participation in related projects or communities.

This is a remote position listed on WFA Digital, the platform for professionals who work from anywhere. Browse more remote jobs across all categories.