In my opinion this is very strange add-on for Mozilla Firefox web browser. At the beginning, I thought that it is some fake or 1st April fool. However, this is the real skin for Mozilla Firefox which causes that it looks the same as Edge.
This project is called Mozilla Edge and it is being developed from November last year. Fortunately, this is not new application, but only a theme which change the appearance of Firefox web browser.
Mozilla Edge skin has been designed by J. Wellkins artist from Canada.
I am really happy that this theme does not any impact on the behaviour of Mozilla Firefox application. I wonder who would like to use decent web browser application from Mozilla which looks like this new failed experiment from Microsoft. However, you would like to see how it looks and works, then HERE you have the theme and installation instruction.
The Mozilla Edge skin requires Firefox 43 version or newer and Windows 10 operating system. Therefore, Linux and macOS/OS X users can not use this. In my opinion, nothing is lost, anyway I would treat this as a curiosity of nature.