Just click the Print... button, you don't need Print preview anymore.

Print selected is still available.

What happens when you click Print or press Ctrl+P? You should get an overlay that combines preview and setup controls.

If your Firefox is managed by an IT department, they might have set a preference to bypass the preview. Does that sound like something worth investigating? You could check here:

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

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

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

(3) If this preference has been customized to true, double-click it restore the default value of false

If there is a lock icon to the left or the preference cannot be changed, then we would suspect an Enterprise Policy or Autoconfig file is locking the preference to true.