Interesting, I'm testing in Basilisk (same dev as Pale Moon) and it also has that. Seems like they are generating a web page with an embed of the flash when viewing it in a new tab. Can't "View Page Source" but if you right-click the border you can "Inspect Element" and see the code:
[html][head][meta name="viewport" content="width=device-width; height=device-height;"][/head][body marginwidth="0" marginheight="0"][embed name="plugin" src="https://i.4cdn.org/f/Everyone%20Is%20Doing%20Their%20Best.swf"
; type="application/x-shockwave-flash" width="100%" height="100%"][/body][/html]
Replaced the tags to go through 4chan's spam filter. Looks like he is trying to hide the border but marginwidth/marginheight doesn't cut it. Pressing CTRL+S saves the swf file and not the html so the browser doesn't treat it exactly the same as a real html page.