There have been a lot of changes to the Firefox 'Print' panel in recent versions of Firefox. This problem is almost certainly the result of those changes. When you save to PDF, are you using the Firefox 'Print' panel with destination set to 'Save to PDF'? If so, as an alternative, try clicking on the 'Save' button on the Print Edit WE toolbar and selecting 'Save As PDF', which gives more control over the page layout using the settings in the Print Edit WE Options dialog.