I'm using SeaMonkey 2.49.4 on openSUSE Tumbleweed (x86_64) installed from the openSUSE Mozilla repository.
Here is the troubleshooting I've done so far:
- I've tried starting my browser in safe mode with a new profile. The video doesn't play this way either. So the problem is probably not with any add-ons or custom configuration in my browser.
- I tried downloading and running the semi-official "Linux x86-64" build of SeaMonkey, but the video won't play this way either. Not even in safe mode with a fresh profile. So the problem is probably not with how my OS is packaging SeaMonkey.
- If I open the Page Info dialog (View->Page Info or Ctrl+I) and select the Media tab, I can see the video listed there, and if I select it and then click on the video, it plays fine. So the problem is probably not a missing codec on my system.
- I've tried opening the same page in Firefox and in Chromium and the video plays fine in those browsers. So the problem is probably something specific to SeaMonkey (or how Instagram treats requests from SeaMonkey).
- I tried getting SeaMonkey to spoof an older version of Firefox by going to about:config and adding keys general.useragent.override.instagram.com and general.useragent.override.cdninstagram.com with the value "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0". The video doesn't play this way either.
- I tried spoofed the user agent string to a more recent version of Firefox: "Mozilla/5.0 (X11; Linux x86_64; rv:66.0) Gecko/20100101 Firefox/66.0". With this setting the page doesn't even load properly.