Simplify Page feature, which when checked in Print Preview window declutters the page for better printing, has been turned off in Firefox 50. Mozilla isn’t going to ship this feature to Firefox users with version 50, three bugs —bug 1306295, bug 1306294, bug 1306299 — are preventing from rolling this feature to release population. Firefox team wants to fix them and planning to release with version 51. But the Simplify Page feature can be turned on easily in Firefox 50 when released, here is how that can be done.
Turning on Simplify Page feature in Firefox 50
1. Visit about:config
2. Find the preference print.use_simplify_page and change its value to true.
For more information, check this bug 1309980 – Disable Simplify Print feature on non-Nightly Linux, and late beta / release.