This is Firefox's upcoming about:performance page (huge improvements)

[harlan4096]

The Firefox web browser will ship with a redesigned and improved about:performance page and related functionality soon.
Mozilla developers are currently working on the redesign and have enabled it partially in recent versions of Firefox Nightly, the browser's cutting edge development version.

The Nightly version sports a new design but limits the display to Energy impact information for the time being. While Firefox users find switches in about:config that turn features on or off in the user interface, none that add more data points to the performance listing seem to work at the time of writing.
Mozilla plans to add memory readings to the about:performance page in Firefox 65.

Tip: Find out what you can do if your browser uses too much memory.

Full reading: https://www.ghacks.net/2018/10/11/this-i...rovements/