Sr. Staff Software Engineer, Browser (New Products)

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. About this team and role Mozilla is seeking experienced browser engineers to join a small team exploring new applications of browser technology. The work combines deep systems engineering, rapid experimentation, and close collaboration across technical disciplines. We are looking for a browser engine specialist who enjoys turning difficult systems questions into working experiments. You will investigate browser behavior, implement changes deep in the engine, and establish which ideas produce meaningful improvements. This is a hands-on individual contributor role with ownership of engine-level investigations and implementation, working closely with the team’s technical lead. The work suits someone who has built or substantially changed browser internals, understands the tradeoffs embedded in those systems, and enjoys applying that knowledge to unfamiliar problems. Your experiments will help inform technical direction. What you'll do: • Design and implement experiments within browser engine subsystems. Investigate how internal data structures and execution behavior affect performance, correctness, and the interfaces exposed to developer tools. • Evaluate ways to represent and expose browser state efficiently and accurately. Identify the tradeoffs in each approach, including information loss, implementation complexity, and compatibility with dynamic web content. • Prototype engine capabilities that support reliable programmatic interaction with web content. Investigate how document structure, layout, navigation, and event handling affect correctness and performance. • Instrument browser behavior and evaluate changes against repeatable tasks. Measure performance and correctness, isolate the effect of each change, and document unsuccessful experiments as clearly as successful ones. Check that gains hold beyond the workloads used to develop the prototype. • Profile the overhead introduced by new capabilities and pursue improvements where measurements justify them. Use AI tools to accelerate coding and analysis, and verify their output through debugging, tests, and measurement. What you'll bring: • Deep hands-on expertise in at least one browser engine subsystem, such as DOM and Web APIs, layout and rendering, accessibility, JavaScript runtimes, or networking and process architecture. You have implemented substantial changes and can trace their effects into neighboring subsystems. • Strong proficiency in C++ or Rust, experience navigating a large systems codebase, and practical knowledge of memory ownership, concurrency, object lifetimes, and performance debugging. You can build the JavaScript and scripting tools needed to exercise and measure your work. • A demonstrated ability to investigate unfamiliar code, form a precise hypothesis, construct a useful prototype, a