Senior Software Engineer, WebExtensions
The WebExtensions platform powers the add-ons ecosystem in Firefox, enabling millions of users to customize their browser while keeping safety and performance at the forefront. As a WebExtensions Engineer, you will help implement new extension capabilities, improve existing APIs, and ensure that extensions remain reliable across Firefox.
As a Software Engineer on the WebExtensions team, you’ll bring a passion for exploring complex systems and creating experiences that balance developer power with user trust. You’ll work deep in the Gecko platform to evolve extension capabilities, and you’ll build UI features in Firefox that make extensions clear and manageable for users. Along the way, you’ll collaborate with experts across Mozilla and help advance cross-browser compatibility and the path toward standardization. Your work will keep extensions safe, powerful, and reliable for developers and millions of Firefox users.
Key responsibilities include implementing and maintaining WebExtensions APIs and Addon Management, balancing power, safety, and performance, as well as user interfaces in Firefox. You will 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 such as code reviews and discussions, and support extension developers by helping diagnose issues and improving tooling and documentation.
The ideal candidate will have 4+ years of professional experience in software development, with strong proficiency in JavaScript, DOM APIs, HTML, and CSS. Familiarity with modern browser architecture and debugging tools, experience with API design and integration, and the ability to navigate and learn from large codebases are essential. Strong communication skills and experience collaborating in distributed teams are also important. Bonus points for experience with Firefox internals, C++ or Rust, familiarity with Android or GeckoView development, knowledge of web security principles, and contributions to open source projects, especially in browser or extension development.
Mozilla offers generous performance-based bonus plans to all eligible employees, rich medical, dental, and vision coverage, generous retirement contributions with 100% immediate vesting, quarterly all-company wellness days, country-specific holidays plus a day off for your birthday, a one-time home office stipend, an annual professional development budget, a quarterly well-being stipend, considerable paid parental leave, an employee referral bonus program, and other benefits that vary by country.
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.