I work at Mozilla, but I'm excited about this because I'm a web developer!

If you've ever tried to use flexbox to align content across multiple rows (like laying out a form with labels), you'd know that it's not a true two-dimensional layout tool. CSS Grid is, and will likely supplant flexbox as a flexbox is merely a one row/column grid.

The inspector is a pretty handy way to see what's going on under the hood, and being able to see the line numbers and named areas is great for visual debugging.