I've read a few different questions with a few different answers, but most of the questions didn't provide much detail and none of the suggestions I've tried have worked — so here I am. Problem first showed up on 6th February, while I was taking and saving screenshots with the built in tool. I didn't think too much of it, just assumed Firefox was giving me a file type choice and continued on my way. Then when I tried saving a few images (both .png and .jpg/.jpeg), the same issue occurred. I've since tried downloading .rar, .zip, and .mp4 files all with the same result. Which leads me to believe that any files I try to download will do the same thing. I've tried deleting mimeTypes.rdf, starting in safe mode (problem still existed), and clearing download defaults in about:config. I got one or two files saved properly after clearing download defaults, but all of a sudden the problem reverted. In Chrome, file type extensions add themselves just fine. I feel like I've provided sufficient information to help anyone who might feel like tying to help solve this thing, but if anything else is needed I'm more than happy to provide it. In the attached screenshots, the test file is an .mp4. After downloading it, I manually added the extension and the video works as normal. EDIT - 11 Feb, 2018: finally got images to upload properly, no longer hosted offsite.