The Print Screen key on the keyboard captures the entire screen as an image and puts it on the Windows clipboard. Then you can paste it into an application that can handle image and either save or print from there. For example, Windows includes a "Paint" program (probably under Accessories) that you can use for this.

If you are just interested in the page displayed in Firefox, there are some handy add-ons to capture not just the visible portion of the page but the entire page. Not sure if you were looking for that.

But how do you print the screen? i know how to do it in Windows 7, but not Windows 10.

There is no literal way to print the entire screen, you have to use an image aware program. Is the problem that you don't have a PrintScreen key on your keyboard? How did you do it on Windows 7?

There are multiple ways to take screenshots in Windows 10, The most popular seems to be by using the Sniping Tool, as it allows you to save a screenshot a little quicker than opening and using Paint.

http://www.howtogeek.com/226280/how-to-take-screenshots-in-windows-10/

http://www.laptopmag.com/articles/capture-screenshots-windows-10