The Secret Button to get 3 Panels of Developer Tools — HTML + CSS + WAY MORE!

Did you know there is a way to see HTML in one pane, CSS in another pane, and a third Developer Tool at the same time in Firefox? Use that third pane to quickly access the Grid or Flexbox Inspectors, the Font Editor, the Animations Tools, Track Changes or more. There’s also a quick way to preview how a page will print.