It is obvious that Firefox needs to download the file to be able to display its content. What changed is that the file might now be saved to the default download folder. What is the current setting for PDF ? Is that open in Firefox or do you get the download dialog and you choose to open the file in Firefox that way? *https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file *https://support.mozilla.org/en-US/kb/manage-downloads-preferences-using-downloads-menu *https://support.mozilla.org/en-US/kb/where-find-and-manage-downloaded-files-firefox