I've never gotten this issue so I'm not sure how to approach it. I'd first try to open the game from newgrounds. Then try another browser and another device if that also doesn't work, just to make sure.
This is the errors in the Firefox browser console:
Uncaught SyntaxError: illegal character U+FFFD JedyneFappyParty 1.4 fix.framework.js.br:1:2
Unable to parse Build/JedyneFappyParty 1.4 fix.framework.js.br! This can happen if build compression was enabled but web server hosting the content was misconfigured to not serve the file with HTTP Response Header "Content-Encoding: br" present. Check browser Console and Devtools Network tab to debug.