In the nightly builds of Firefox that will form the basis of the Firefox 81 release, new implementation of the preview interface before printing. The new preview interface is notable for opening in the current tab and replacing existing content (the old preview interface opened a new window), i.e. works similarly to reader mode. Tools for setting the page format and print options have been moved from the top to the right panel, which also has additional options, such as controlling whether headers and backgrounds are printed, as well as the ability to select a printer. About:config provides a print.tab_modal.enabled setting to control whether the new mode is enabled.
Source: opennet.ru