First, this is on a Nexus 7 (2012) with the latest version of Android 5.1 and Firefox for Chrome 37.0.2.

When I download a video file in Chrome it shows up, for example, as a 100MB file MP4 file in Downloads. But if I do it in Forefox, it shows up as a 100MB htm file. I download the files so I can watch in the gym, which doesn't have good WiFi. I want to use Firefox because the latest Chrome for Android is locking up my phone, but I can't use Firefox for videos. I want to be able to turn off my Wifi in the gym and watch the videos. Is there a way to change how Firefox downloads files?