We’ve reported using Reader Mode Firefox allows you to print pages better without clutter or ads, till now you need to visit about:config and change print.use_simplify_page preference to true for that, which results in the addition of simplify page checkbox on the print preview toolbar. Mozilla has enabled this feature by default in Firefox 50 Nightly without you needing to visit the about:config. 

Mozilla enables Simplify Page feature by default for Print Preview

To test this feature right away, in the latest version of Nightly, visit any webpage or article that has a lot of text, click on Hamburger menu  and select Print for the print preview screen to appear, if the page you’re trying to print supports reader mode, you’ll see Simplify page check box activated, enable it and wait for reader mode to prepare the page for printing.

Simplify page

In Nightly, print preview for a page before and after selecting Simplify Page checkbox]

READ: Opera 26 includes a built-in Print Previewer for Pages

If the page doesn’t support reader mode, you’ll notice simplify page checkbox grayed out on the toolbar of print preview. Check the above screenshot for before and after selecting simplify page checkbox.