Hi, in the old print experience, Simplify Page applied Reader View to the page displayed in the Print Preview panel. With the new print experience, you can click the Reader View icon in the address bar to put the page in Reader View before calling up print and it should carry over to the print preview automatically.

We're all learning about this new print experience in real time, and improvements are expected in future versions. If it doesn't suit your needs, it's possible to roll back the change at least for now:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste tab_mo and pause while the list is filtered

(3) Double-click the print.tab_modal.enabled preference to switch the value from true (new experience) to false (classic)

To add some more information to what jscher2000 said, a "Simplify page" checkbox should hopefully be added to the new user interface in Firefox 87. That work is tracked here if you want to monitor progress: https://bugzilla.mozilla.org/show_bug.cgi?id=1666247