Aethan contributed to mozilla/gecko-dev by improving build reliability, code clarity, and test robustness in C++ and JavaScript. He resolved a GCC build error in Tracks::ResolveIntrinsicSizeForNonSpanningItems by updating type declarations, ensuring compatibility and reducing build failures. Through code refactoring, he removed redundant mozilla qualifiers from EnumeratedArray, enhancing maintainability without altering functionality. Aethan also improved test configurations for CSS grid track sizing, enabling multi-pass verification and reducing timeouts. His work demonstrated strong debugging, code review, and testing skills, delivering targeted, well-scoped changes that addressed specific pain points in the build and testing processes within a short timeframe.