Unfortunately there's nothing I can do about PDF files for now due to changes in Firefox that mean extensions can't interact with the PDF viewer (see bug 1454760 - https://bugzilla.mozilla.org/show_bug.cgi?id=1454760). Some people have been able to work around this by using the PDF viewer at https://mozilla.github.io/pdf.js/web/viewer.html and then opening a different file from the toolbar in the upper right.