I've had the in-browser PDF viewer enabled as long as I've used Firefox. All of a sudden this past week, the PDFs I try to open are automatically downloading to my C drive , even though I didn't change any settings. Sometimes the download is instant, and other times the new tab (where the PDF viewer would typically show the PDF) opens for a split second and then automatically closes and downloads the PDF. In preferences, the in-browser PDF viewer is still enabled. This is very frustrating and somewhat worrying, since I don't like files to automatically download. I know I can require a window to specify download location, but still, the PDFs should be opening in my browser without downloading anyway. I'm using Mac OS 10.11.6 and Firefox 58.0.2. I know this isn't an overarching issue with my computer because in Google Chrome, PDFs still open in a new tab in the browser as they always have, and as they should. This issue is isolated to Firefox. Hope to hear back ASAP with a solution. Thanks!