Deep expertise in a browser engine subsystem (e.g., DOM, layout, JavaScript) with hands-on experience in implementing changes.
Strong proficiency in C++ or Rust, with practical knowledge of systems programming concepts.
Ability to investigate unfamiliar code and construct useful prototypes based on hypotheses.
Strong understanding of web platform behavior and compatibility issues.
Experience with profiling, tracing, and regression tests, grounded in reproducible evidence.
Curiosity about AI agents and experience with coding tools related to AI.
Responsibilities
Design and implement experiments within browser engine subsystems to assess performance and correctness.
Evaluate and represent browser state efficiently, identifying tradeoffs in implementation.
Prototype engine capabilities for programmatic interaction with web content.
Instrument browser behavior, measure performance, and document both successful and unsuccessful experiments.
Profile overhead from new capabilities and pursue performance improvements using AI tools.
Benefits
Generous performance-based bonus plans for eligible employees.
Rich medical, dental, and vision coverage.
Generous retirement contributions with immediate vesting.
Quarterly wellness days for all employees.
Country-specific holidays plus an additional day off for your birthday.
One-time home office stipend for remote work setup.
Annual professional development budget for skill enhancement.
Quarterly well-being stipend to support employee health.
Considerable paid parental leave for new parents.
Employee referral bonus program to encourage team growth.
Full Job Description
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, 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.
Nice to have:
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
What you'll get:
Generous performance-based bonus plans to all eligible employees - we share in our success as one team
Rich medical, dental, and vision coverage
Generous retirement contributions with 100% immediate vesting (regardless of whether you contribute)
Quarterly all-company wellness days where everyone takes a pause together
Country specific holidays plus a day off for your birthday
One-time home office stipend
Annual professional development budget
Quarterly well-being stipend
Considerable paid parental leave
Employee referral bonus program
Other benefits (life/AD&D, disability, EAP, etc. - varies by country)
Hiring Ranges:
US Tier 1 Locations
$195,000-$260,000 USD
US Tier 2 Locations
$179,000-$239,000 USD
US Tier 3 Locations
$166,000-$221,000 USD
About Mozilla
Mozilla is a global community of technologists, thinkers, and builders working together to keep the internet open and accessible to all. The company is best known for its flagship product, the Firefox web browser, which is used by millions of people around the world. In addition to its browser, Mozilla also develops a range of other products and services, including a mobile operating system, a password manager, and a virtual private network (VPN) service.
