Video stuck loading on Firefox

I'm currently building a site so it's still on the temp url. When I view it on Firefox, videos regularly do not load. It will be stuck with a black screen and loading icon. I've not had this issue before so I'm not certain if it's happening because of the temp URL/password protection or something else? I've loaded fallback images and the videos have all been compressed to roughly 3MB. This is happening for videos used as section backgrounds and for video blocks. It sometimes happens on Chrome but seems to be a consistent issue with Firefox. In googling the issue, the only thing I could find was a suggestion to turn off Firefox's "enhanced tracking protection" which doesn't seem to have made any difference. I'm wondering if it will resolve once the site is on the permanent URL and the lockscreen has been removed? I should note that when I first open a page, sometimes it loads fine. But if I navigate to a different page, then return, it is consistently stuck on the loading screen. Any insight is appreciated. Thanks 

I already reported this problem to squarespace months ago. When you upload a video to a Squarespace website (instead embed it), the video usually works for once but then, it presents continuously this problem (in Firefox). In Chrome, usually seems to work better. If the visitor clean the Firefox history, the video comes back to normal but in the very next time he visit the page, the problem shows again. It's an annoying problem, unfortunatelly gives an unprofessional impression but Squarespace didn't correct this bug.