Maybe use the Windows Print feature instead of Firefox Print. Open the Charms Bar and click devices. Click on a printer and then put the settings. Press print.

Many print buttons launch a new window with a reformatted page more suitable for printing. If the problem is with the new windows, it could be the Firefox pop-up blocker, or a pop-up blocker in an extension or external security software.

To manage the built-in popup blocker, see: Pop-up blocker settings, exceptions and troubleshooting.

To determine whether an extension is a likely culprit, you could test one of the problem sites in Firefox's Safe Mode. That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

I just noticed you wrote "only empty papers" in your question. Did you see this support article: Fix printing problems in Firefox?