Staff Software Engineer, Browser (New Products)
Firefox · Remote US
About this role
**Why Mozilla** Mozilla is a non-profit-backed technology company that has shaped the internet for the better over the last 25 years. We build pioneering, privacy-minded products like Firefox, and we’re focused on making the internet better for people—not companies. Mozilla is wholly owned by the non-profit 501(c) Mozilla Foundation, so we’re not beholden to shareholders—only to our mission. We design, build, and distribute open-source software with thousands of volunteer contributors and collaborators worldwide. **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’re looking for a browser engine specialist who enjoys turning difficult systems questions into working experiments. You’ll investigate browser behavior, implement changes deep in the engine, and determine 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. **What you’ll do** - Design and implement experiments within browser engine subsystems, investigating how internal data structures and execution behavior affect performance, correctness, and developer tools interfaces. - Evaluate ways to represent and expose browser state efficiently and accurately, including tradeoffs around information loss, complexity, and compatibility with dynamic web content. - Prototype engine capabilities that support reliable programmatic interaction with web content, investigating document structure, layout, navigation, and event handling. - Instrument browser behavior and evaluate changes against repeatable tasks—measuring performance/correctness, isolating each change’s effect, and documenting unsuccessful experiments clearly. - Profile overhead introduced by new capabilities and pursue improvements where measurements justify them. Use AI tools to accelerate coding and analysis, and verify outputs via debugging, tests, and measurement. **What you’ll bring** - Deep hands-on expertise in at least one browser engine subsystem (e.g., DOM & Web APIs, layout & rendering, accessibility, JavaScript runtimes, networking & process architecture), with experience implementing substantial changes and tracing effects into neighboring subsystems. - Strong proficiency in C++ or Rust, experience navigating large systems codebases, and practical knowledge of memory ownership, concurrency, object lifetimes, and performance debugging. Ability to build JS/scripting tools to exercise and measure your work. - Ability to investigate unfamiliar code, form precise hypotheses, build useful prototypes, and interpret results—separating implementation effects from variability in websites, workloads, or model behavior. - Strong understanding of web platform behavior and compatibility, including reasoning about how document structure, script execution, rendering, and user interaction relate—especially when internal browser state is hard to expose accurately. - Experience with profiling/tracing/regression or conformance tests, and a habit of grounding performance claims in reproducible evidence. - Curiosity about AI agents and willingness to work directly with agent harnesses and coding tools, connecting low-level implementation choices to what agents can
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