Senior Staff Software Engineer

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.

• 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.

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.

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.

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.

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.

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.

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.

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.

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.