If you are using Firefox 55 or earlier, then the 'Preview' button on the Print Edit WE toolbar opens the 'Print' dialog, rather than switching to Firefox's print preview window. In this case, you will have to open Firefox's print preview window using 'File > Print Preview' on the Firefox main menu bar. Before doing that, click on the Print Edit WE button to suspend editing, and the Print Edit WE toolbar in the page will be hidden.

If you are using Firefox 56 or later, the the 'Preview' button on the Print Edit WE toolbar switches to Firefox's print preview window.