I'm not sure what you mean by "XUL documents" "on a website." I think of XUL documents as internal Firefox pages, and it's true that it may not be possible to print those. Is that what you mean? Are there particular pages that are a problem (you could mention the address)?
Hi Carrieshaw, it's a little hard to diagnose without a URL.
Are you saying you can preview the page but not print from preview, or you can't preview at all?
Do you know whether it is an ordinary web page or a PDF displaying in Firefox's PDF viewer? The PDF viewer shows a black background behind the document and has a standard toolbar at the top of the document.
If it's a PDF, you could try using the download button on the viewer toolbar (it's a downward-pointing arrow) to open the PDF in Adobe Reader (outside Firefox) or you could try using the Adobe viewer in Firefox instead of the built-in viewer. This article has the steps for that: How to disable the built-in PDF viewer and use another viewer.