Sr. Staff Software Engineer, Browser (New Products)

Mozilla is a non-profit-backed technology company that develops open-source products such as Firefox and works to improve the internet for people. The Senior Staff Software Engineer will investigate browser behavior, implement and evaluate changes within browser engine subsystems, and develop experiments involving web content interaction, performance, correctness, and AI-assisted tooling.

Responsibilities:

• 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

Requirements:

• 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, and interpret the results. You can separate the effects of your implementation from variability in websites, workloads, or model behavior
• A strong understanding of web platform behavior and compatibility. You can reason about how document structure, script execution, rendering, and user interaction relate, including cases where the browser's internal state is difficult to expose accurately to another system
• Experience with profiling, tracing, regression tests, or conformance tests, and a habit of grounding performance claims in reproducible evidence. You can explain both the benefit of an optimization and the circumstances in which it fails
• Curiosity about AI agents and a willingness to work directly with agent harnesses and coding tools. You can connect a low-level implementation choice to the information an agent receives and the work it must do next
• Contributions to Gecko, Servo, Blink, WebKit, or another browser engine
• Experience maintaining an open-source project
• Technical leadership in a small team building a new product
• Experience with accessibility infrastructure, browser security, profiling, or automated evaluation of agent workflows