On certain websites, Firefox would not open PDF files in-browser using PDFJS: I get a dialog box where the only possibilities are "Cancel" and "Save the file" (not even opening the file in an external reader). I've tried various solutions found online: - delete the "mimeTypes.rdf" in the profile folder with Firefox closed - make sure "pdfjs.disabled" is set to "false" in the "about:config" page - set the application preference to "Preview in Firefox". Actually, even setting the preference to "Use external viewer (okular)" won't change this "save/cancel" dialog box... It seems to be pretty random when the preview works and when not. The latest example I have is a gitlab instance (with restricted access, so I cannot share it with you unfortunately).