Hi Thank you for your question. I think that the answer could be one of two options; It could be due to the number of tabs that you have open causing a little be of performance strain. Yes, Firefox can have open many many tabs, but this could be stopping the document from creating properly. and/or I do know that some *.pdf files are created in such a way that Firefox cannot display them properly. For example, in the UK, the Companies House website *.pdf files will not open in Firefox, but they (not Mozilla) are looking at how the files are created and to make them more standards compliant. Try opening the file with no other tabs open and see if that works. If not, see if it is just files from one site that is causing this issue. I hope this helps, but if not please come back here and we can look into a different solution for you.