Mozilla Firefox Developer Edition for Mac

From Mozilla: Firefox is the only browser with tools built specifically for building and designing with CSS Grid. Firefox Developer Edition automatically sends feedback to Mozilla. These tools allow you to visualize the grid, display associated area names, preview transformations on the grid and much more. The new Firefox DevTools are powerful, flexible, and best of all, hackable. This includes a best-in-class JavaScript debugger, which can target multiple browsers and is built in React and Redux.