You need an account for that so the link is not possible ....
firefox --safe-mode to temporarily run with all add-ons disabled? If the page/site allows you to download the PDF, you might also try that and using a dedicated PDF viewer to display the document (or just try Firefox again; security settings for file URLs may be less restrictive compared with http/https). The last option would be to examine the browser's console log for any information you might be able to understand (Menu → More Tools → Browser Console). I've had issues in the past with Firefox and PDF files on certain sites as well. In my case, I eventually gave up and switched to Chromium to view the file. However, I love my flaming canid too much to stop using it just because it cannot handle an occasional PDF file since it works great for other PDF files! pdfjs.enablescripting is true by default (yes, Mozilla is a lost cause). Thus, if there's javascript in the PDF, e.g. form-related pages, Firefox will interpret it for you. about:config. Search for pdfjs.enablescripting, and toggle it, i.e. true -> false, or vice versa. Check the PDF file, and let's see what will happen.